Hemicyanine‐doped silica film was fabricated by sol–gel technique. Time evolution of structural changes in hemicyanine and their influence on the second‐order optical nonlinearity were investigated through UV‐visible absorption and second‐harmonic generation (SHG) measurements. We found that the conversion of protonated hemicyanine to monomeric and aggregated hemicyanine occurred within 4 h after film deposition. During this period of time the SHG signal continued to rise and finally reached the maximum. The growth of optical nonlinearity was mainly due to deprotonation of hemicyanine molecules. Blueshift of the absorption peaks for both aggregate and monomer were found during the drying of the film. ©1996 American Institute of Physics.