Phase separation driven by photochemical reactions was examined for two types of polymer mixtures: one in the liquid and the other in the bulk state. In the former, monomer of the ?rst polymer was photo-polymerized and simultaneously cross-linked in the presence of the second polymer in the liquid state, whereas in the second system, a polymer was photo-cross-linked in the presence of the second polymer in the bulk state. A combination of measurements techniques including FT-IR, UV?VIS spectrom- etry, confocal microscopy, light scattering and Mach?Zehnder interferometry was utilized in order to elucidate the stimulus?response behavior of these polymer networks. The shrinkage of the mixtures occurring during the photo-polymerization process was in situ observed under a laser-scanning confocal microscope (LSCM), whereas the deformation process during the cross-link process in the bulk state was in situ followed by using Mach?Zehnder interferometry (MZI). The correlation between the emergence of the hexagonal-phase morphology and the reaction-induced shrinkage in the polymerizing mixtures was discussed from the reaction kinetics and the LSCM data. For the mixture in the bulk state, MZI data provide a reasonable explanation for the mechanism of the anomalous light scattering from the reacting polymer blends. These experimental results are ?nally discussed in the context of systems with compet- ing interactions.