Research of a membrane's polymeric material obtained by photoinitiated radical polymerization of oligocarbonate methacrylate and butyl methacrylate (BMA) was conducted by the method of reversed gas chromatography. The presence of defects of two types in the polymer was confirmed: as gaps between grains, and spongy interlayers. It has been established, that the relative size of gaps between grains more than in 1.5 times exceeds the size of interlayers between grains. The selective separation of substances proceeds through interlayers between grains. The size of interlayers (polymer's T-g) depends on structure of an initial composition. Increase of the BMA contents in an initial composition leads to polymer's T, increase, condensation of interlayers between grains, and increase of selectivity of the appropriate membrane. This dependence has an extremal character.