Semi-interpenetrating polymer networks (semi-IPN) of styrene-(chloromethyl)styrene copolymer (P(S-stat-CMS)) and poly(vinyl methyl ether) (PVME) were prepared by in situ photo-crosslinking of miscible P(S-stat-CMS)/PVME (50/50) blends. To selectively cross-link the P(S-stat-CMS) component, the CMS segments of this copolymer were chemically labeled with anthracene. Upon irradiation at 365 nm, anthracene undergoes photodimerization and induces network formation of P(S-stat-CMS) chains in the blend. The miscibility as well as the morphology of the resulting semi-IPN critically depends on the competition between the cross-linking reaction kinetics and the polymer diffusion driven by thermodynamic instabilities accompanying the reaction. Photo-cross-linking of P(S-stat-CMS) at a temperature close to the glass transition region of the blend provides a transparent semi-IPN with a broad T-g. On heating, these semi-IPNs undergo phase separation. The phase domains are restricted to the nanometer scale as revealed by small-angle X-ray scattering. The phase-separated semi-IPNs exhibit iridescence when obtained during the photo-cross-linking reaction carried out in the one-phase region close to the binodal temperature. Phase-contrast optical microscopy combined with digital image analysis showed that these particular semi-IPNs possess lamella-like structures in the micrometer scale. These preliminary results indicate that multiphase polymers with a wide variety of controllable ordered structures ranging from the nanometer to micrometer scale can be obtained by using in situ photo-crosslinking of miscible polymer blends.