Sonia met the young painter Robert Delaunay in 1907. After she divorced Uhde, they were married in 1910 and moved to an apartment in Rue des Grands-Augustins.
Robert’s early paintings had been influenced by impressionism and cubism but he and Sonia turned their attention jointly towards abstraction. Following the nineteenth-century chemist Michel Eugène Chevreul, who studied how the perception of colours seems to change when they are placed alongside each other, Sonia and Robert developed a theory of simultaneous colour contrasts which they called simultanism.
As well as painting, Sonia began to apply these theories to a variety of forms. Shortly after the birth of their son Charles, she made him a patchwork cradle cover, bringing together the traditional techniques used by Russian peasant women with modernist abstraction.
Sonia’s striking designs for cushions, lampshades and other objects filled their apartment, helping to establish their home as an important meeting place for artists and poets.
In 1913 Sonia befriended the poet Blaise Cendrars. Together they created Prose on the Trans-Siberian Railway and of Little Jehanne of France 1913, in which the poet’s fictional recounting of a journey from Moscow to Paris is interpreted through Sonia’s abstract stencil illustrations.