A summary of in situ dynamic studies of three distinctly different chemically selective films is presented. A chemically selective film is a crucial component of a chemical sensor. Spectroscopic ellipsometry was used to determine film thickness and refractive index changes in time as these films were exposed to aqueous electrolyte solution. These three films are presented as case studies illustrating the usefulness of dynamic spectroscopic ellipsometry in deciphering a range of film behaviors-from film robustness to film failure. The three films studied were Nafion-SiO2, PDMDAAC-SiO2 and PVTAC-PVA. The first two of these represent rigid silica (SiO2) networks in which an ion exchange polymer is immobilized. The third film consists of an all polymer based composite with an ion exchange polymer entrapped by cross-linking the host polymer PVA. Each of these three films exhibited different dynamic behavior on soaking in aqueous electrolyte solution. The Nafion-SiO2 film remained essentially unchanged over time and represents a robust useful sensor film. The PDMDAAC-SiO, film underwent large changes and ultimately catastrophically failed when exposed to solution for a long period of time. The PVA-PVTAC film swelled by approximately a factor of 1.5 when exposed to solution but remained intact, fully functional and bound to the substrate surface over time. Knowledge of sensor film properties is important in understanding how a chemical sensor works or fails and in the modeling of chemical sensor response. In the case of film failure, determination of the underlying reasons can often be deduced from dynamic measurements of ellipsometry parameters. (C) 2004 Elsevier B.V. All rights reserved.