Trans-cis photoisomerization of an azobenzene derivative chemically labeled on polystyrene (PSAzo) was utilized as a tool to reversibly drive the phase separation of a PSAzo/poly(vinyl methyl ether) (PSAzo/PVME) blend under various experimental conditions. By using light scattering and UV-vis spectrophotometry, it was found that trans -> cis photoisomerization of azobenzene took place upon irradiation with 365 nm UV light, leading to an increase in solubility of the mixture. The enhancement in the affinity between the two polymers arises from an increase in the hydrophilicity of azobenzene chemically labeled on the PSAzo chains upon trans -> cis isomerization. Effects of the affinity between the cis-PSAzo and PVME oil the phase separation kinetics of the PSAzo/PVME blends were also revealed by combining experiments on thermally induced phase separation and light-induced dissolution. In contrast to the phase separation induced by a temperature drop where the dissolution of the periodic structures with shorter wavelengths is dominating, it was found that structures with longer wavelengths are predominantly dissolved during the reverse quench induced by irradiation with 365 nm UV light. These experimental results not only reveal a unique mode-selection process driven by UV light but also suggest a tool to control morphology of photoreactive polymer mixtures.