Phase separation driven by a reversible photo-cross-linking reaction was investigated by using anthracene-labeled polystyrene/poly(vinyl methyl ether) (PSA/PVME) blends. Upon irradiation with 365 nm ultraviolet light, anthracene moieties undergo photodimerization, leading to phase separation of the two polymers. On the other hand, the homogenization process of the phase-separated blend was induced by irradiation with 297 nm which converted photodimer back to anthracene monomer, thus de-cross-linking the PSA networks in the mixture. The kinetics of phase separation and the corresponding homogenization processes driven by these two UV wavelengths were followed in situ under a light scattering instrument equipped with a UV light source. Unlike the conventional kinetics of phase separation in nonreacting mixtures, it was found that the scattering peak shifts toward the side of larger wavenumber as phase separation proceeds under 365 nm light. The local elastic deformation of the blend monitored along the course of irradiation by Mach-Zehnder interferometry reveals a strong correlation between the cross-link-induced elastic deformation and the shift of the scattering peak observed during irradiation. On the other hand, the homogenization process of the cross-linked blends was observed upon irradiation with 297 nm. The scattering intensity decreases while the position of the scattering peak remains unchanged with irradiation time. These experimental results do not only provide basic information for modeling reaction-induced phase separation but also would suggest a method for recycling multicomponent polymer blends by taking advantages of UV-induced reversible phase separation.