Dye‐in‐polymer (DIP) films consisting of soluble Pr4VOPc (vanadyl phthalocyanine having four propyl substituents) dye and polymethylmethacrylate (PMMA) were prepared by spin‐coating process. Pr4VOPc precipitates as a glassy phase in pure PMMA matrix during the film formation. Upon exposure of these DIP films to organic solvent vapor, stable crystalline Pr4VOPc, which has stronger absorption in the near‐IR region and larger nonlinear refractive indexn2than the glassy phase, forms. The organic solvent vapor promotes the solubilization and crystallization of the dye, plastication of the polymer chains, and migration of the dye. The nonlinear refractive indexn2and optical spectra of the DIP films were measured, and the change rule has been discussed in terms of electron transition and molecular packing of Pr4VOPc. ©1995 American Institute of Physics.