Dissolution and removal of photoresist films is performed numerous times during the fabrication of electronic, optical, and mechanical devices and integrated circuits. Information regarding changes in refractive index and film thickness is of interest for the design and modeling of such processes. Spectroscopic reflectometry is a cost-effective technique that has been used to monitor film thickness, but has been limited to applications in which the film refractive index is constant during dissolution, and thus has a known value. This paper extends conventional spectroscopic reflectometry analysis to account for refractive index changes in films during processing. Results obtained by this approach correlate well with spectroscopic ellipsometry measurements. This technique is applied to the dissolution of photoresist in aqueous base solutions, swelling of photoresist in solvent vapor, and removal of photoresist in water-alcohol mixtures, and can similarly be applied to other thin film applications. In addition, the optical properties of fluids such as aqueous and non-aqueous solvents, supercritical or subcritical mixtures, and gas-expanded liquids, which can be used to process thin film materials, can be monitored using spectroscopic reflectometry.