Phase separation of polystyrene derivatives (PSS) and poly(vinyl methyl ether) (PVME) mixtures was induced in the one-phase region by using photoisomerization of trans-stilbene moieties labeled on the PSS chains. The phase separation was monitored by phase-contrast optical microscopy and was analyzed using digital image analysis under various experimental conditions such as temperature, light intensity, and the blend composition. During the reaction, the spontaneous pinning of the phase separation process was observed as predicted by the linear stability analysis of the time-dependent Ginzburg-Landau equations (TDGL) proposed recently for chemically reacting systems. The photoisomerization in the one-phase region of the blends does not follow the mean-field kinetics and can be well expressed by the Kohlrausch-Williams-Watts mechanism. By varying the annealing time and the reaction rates, it was found that the elastic stress resulting from the reaction inhomogeneity plays a key role in the emergence of lamella-like morphology. The correspondence between the phase separation of polymer blends accompanied by a reversible reaction aid the microphase separation of diblock copolymers is discussed by comparing these experimental data with recent simulation using the TDGL equation for reacting mixtures. Finally, the morphology control based on phase separation induced by reversible reactions is discussed in terms of a wavelength selection process.