A chemically selective film, usually of optical quality, is a key component of optical and electrochemical sensors. We have examined the dynamics of various thin selective films by spectroscopic ellipsometry and attenuated total reflectance (ATR) spectroscopy. Spectroscopic ellipsometry provided a non-invasive method for measurement of optical constants (n(λ), refractive index; k(λ), extinction coefficient) and thicknesses of thin selective films which were conditioned in time by solvent and penetrant mass transport. The methods we have developed allowed us to characterize film dynamic response, stability, and chemically specific mass transport even to the point of quantitatively modeling both transport and structural changes. The ATR spectra of thin, highly absorbing films are distorted both by reflectance and interference phenomena. Enhanced absorbance was observed under film leaky mode conditions.