The phase behavior of trans-stilbene-labeled polystyrene and poly(vinyl methyl ether) mixtures was investigated by using small angle neutron scattering. It was found that the binary interaction parameter, c, obtained by analyzing the scattering functions with the random phase approximation theory increases as the trans-stilbene is converted to the cis-isomer. The dependence of c on the fraction of trans- and cis-isomers in the blend was experimentally obtained and discussed using random copolymer theory. Furthermore, the temperature dependence of c varies linearly with the reciprocal temperature. These experimental results directly reveal the possibility of controlling the miscibility of polymer blends by photochemical reactions such as photoisomerization.