This article reports an in situ approach to studying acid loss from polymer (films of the type used in microlithiography. We developed a method which makes use of the dye coumarin 6 (C6) which has absorption characteristics which change dramatically as a result of acid-induced changes in its prototropic forms. Acid loss from polymer films containing C6 is accompanied by its deprotonation, which is signaled by a change in the absorption of the film. This technique provides kinetic data describing acid loss from polymer films. Acid loss is a function of both acid volatilization from sites at the surface of the film as well as diffusional processes within the film. Under controlled conditions, acid loss obeys first-order kinetics, and the half-lives, tau(1/2), of acids in phenolic and acrylate polymer films were determined. We studied the effects of temperature, acid structure, polymer structure, and T-g on the tau(1/2) values obtained.