This work reports the feasibility of polybutadiene (PB) cross-linking under UV irradiation in the presence of a linear polymer, cellulose acetate (CA), to form semi-interpenetrating polymer networks at the air–water interface. The thermodynamic properties and the morphology of two-dimensional (2D) CA/PB blends are investigated after UV irradiation and for a wide range of CA volume fractions. A contraction of the mixed Langmuir films is observed independent of the composition, in agreement with that recorded for the individual PB monolayer after cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic CA/PB mixed film. From Brewster angle microscopy observations, the PB network synthesis does not induce any morphology change at the mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect of PB cross-linking on the vertical structure of CA/PB mixed films. For all studied compositions, significant thickening of the films is evidenced, consistent with their contraction ratio. This thickening is accompanied by a partial expulsion of the PB toward the film–air interface, which is attributed to the hydrophobic character of the PB. This phenomenon is stronger for films rich in PB. In particular, the structure of the PB-rich film undergoes a transition from vertically homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating polymer networks can thus be synthesized at the air–water interface with a morphology that is strongly influenced by the polymer–polymer and polymer–environment interactions.