The reflectance of a thin film changes upon immersion in a transparent dielectric liquid. By use of spectrophotometry, the changes in reflectance at the wavelengths where maximum or minimum reflectance occurs have been characterized for a variety of optical combinations including both transparent and absorbing films and substrates. The complications and ambiguities of interpretation for each system and the effects of multilayer structure have been examined. Reversal of the direction of observation is discussed as one means to distinguish between absorption and inhomogeneity in the film.