The artists' theoretical ideas on this issue were summed up in 1931, in a book entitled Kompozycja przestrzeni. Obliczenia rytmu czasoprzestrzennego ("Spacial Composition. The Calculation of Space-Time Rhythm") that she wrote together with Strzemiński. The book was mostly an outline of the principles of Unism, but it went beyond painting, expanding into sculpture and architecture.
At that time Kobro and Strzemiński were already members of the A.R. group, along with Stażewski and the poets Julian Przyboś and Jan Brzękowski. The group took on many important projects, including the creation of the International Collection of Modern Art in the J. and K. Bartoszewicz City Museum of History and Art in Łódź (the Museum of Art). The festive opening of the museum's modern art room took place in 1931.
In the same year the couple moved to Łódź from their home in Koluszki, where they had been living. They were still making a living by teaching, and they led an active artistic life. They accepted an invitation to participate in "Abstraction-Création" and "Internationaler Ring der neuen Werbegestalter" (it is probable that Kobro participated as well, though it is not certain). They also exhibited frequently in Poland, and they continued to write. One of the most important and talked-about exhibitions of the time took place in 1933 at Warsaw's Instytut Propagandy Sztuki (Institute for the Propaganda of Art), also featuring artists from the L'art contemporain circle and the Grupa Krakowska. Wacław Husarski understood the work, writing:
Stażewski, Strzemiński and Ms Kobro are the most radical in their quest to express contemporary reality, finding the solutions for their abstract problems in linear, coloured, textured or spatial compositions just as a technician mathematically solves the problem of a machine's construction. In these works [...] the tragic element of art is fully exposed, as it strives to suit the mathematical and technical mind-set of the contemporary era.
Apart from her artistic work which included the "biological" Kompozycja przestrzenna (Spatial Composition of 1933), Kobro was also an active organizer. She worked for the Związek Zawodowy Polskich Artystów Plastyków (Trade Union of Polish Artists) - a group of Strzemiński's and Karol Hiller's followers – where she served as vice-member of the board and, from 1935, as treasurer.
In 1936 Kobro and Strzemiński had a daughter whom they named Nika. The child was sickly and required constantly care, making it difficult for Kobro to practice her art. At the end of that year she exhibited her work for the last time, four paintings entitled Pejzaż morski (Seascape) probably painted in Chałupy on the Hel Peninsula, where the couple had spent their holidays. Soon afterward she repeated the main thesis of her credo in "Głos Plastyków" (1937, no. 1-7):
Sculpture is a part of space. That is why the condition for its organic character is its relation with space. Sculpture should not be a composition of form enclosed in mass, but rather an open space construction where the inside of the compositional space is connected with the outside. Rhythm is compositional unity. The energy of subsequent shapes in space produces a spatiotemporal rhythm.
The task of art, she wrote,
is to work toward the victory of higher forms of organisation of life. Art's role is the production of form in the interest of social utility.
This is the position from which Kobro attacked the "official and bureaucratic art" of Alfons Karny, the "caligraphic stylisation" of Jan Szczepkowski and the sculpture of Stanisław Szukalski, writing that its "'national character' reminds one of contemporary Nazi sculpture."
Kobro spent the years leading up to World War II mostly keeping house and taking care of the child - it is not known whether she created any new works during this period. The family had to move frequently during the war. In 1940 the Strzemińskis returned to Łódź, only to find that, as a result of their travels, they had lost the work that had been left in the cellar of their pre-war apartment; the occupants that came after them had thrown the artwork away. Kobro was the primary victim of this "cleaning", and she was able to find only some of her works on a scrap-heap. Soon afterward, in 1945, she destroyed some of her wooden sculptures herself, burning them in order to cook a meal for her hungry daughter.
After the war Kobro's life became less and less stable. Strzemiński attempted to deprive her of the right to care for their daughter (he failed), and she was prosecuted for "deviating from the Polish nationality", sentenced to six month in prison and only released after an appeal. She also fell seriously ill with a malady that would later be the direct cause of her death. Among her later works are the plaster Akty (Nudes) in 1948, and crayon landscapes from Siemiatycze. Akty act as a brace for Kobro's sculpture. Janusz Zagrodzki compares them to her Akty of the 1920s, emphasising the greater expressive power of these last works, their monumentality and their dynamics. He draws attention to their formal complication and elaboration, derivatives of the courageous deformation of the human body. But he also underlines their sensuousness, also a characteristics of her earlier works, although the later sculptures are more concise, simpler, "almost Classical" (Zagrodzki). A convoluted path can be traced between those first works and the later ones, a path that traces the artist's evolving social and aesthetic consciousness.
Kobro's aesthetic ideas, like Strzemiński's, encompassed not only the work of art itself, but also art's presence and function outside of the artistic sphere. Strzemiński wrote that in suprematism, "the background is a constructively passive factor," whereas he himself was interested in "the absolute communion of background and shape in one original whole" – in other words, "post-suprematism" (unism). Kobro's ideas were similar, her ambition being to "compose space." Both refused to accept the separation between "pure" and "applied" art. Nor was Kobro interested in intuitive or visionary work, trusting primarily in the rigours of arithmetic and logic.
She aimed to create a system that would logically combine the dimensions of all shapes, where the arrangement of even the tiniest elements would be precisely calculated, Janusz Zagrodzki writes.
What was important for her was the presentation of her sculptures and the colours of each particular part (in Rzeźby Abstrakcyjne ("Abstract Sculptures") she respected the natural colour of her material, sometimes adding black and white; in Kompozycje przestrzenne ("Spatial Compositions") she used yellow, red, blue, black and white).