Chemical analyses of vapors desorbing from chemically amplified resist films and kinetic analyses of deprotection, vaporization, and film densification during postexposure thermal processing provide important information on the volatilization of resist components, available decomposition pathways, and mechanistic details of image formation. We have made several types of in situ measurements to characterize imaging chemistry and physics during postexposure bake (PEB). Volatile products from the thermal and acid-catalyzed deprotection of poly(tert-butoxycarbonyloxystyrene) (PTBOCST) to form poly(hydroxystyrene) have been identified by mass spectrometry. The results show that two reaction pathways are present with markedly different chemistries and that the generally accepted deprotection mechanism oversimplifies what is actually happening in the film. Using infrared spectroscopy and spectroscopic reflectometry, the relative rates of deprotection, product volatilization, and film densification of PTBOCST homopolymer and copolymer resists during PEB were assessed. These data rule out any significant accumulation of excess free volume or marked plasticization of the resist film during normal PEB, providing evidence against a transient enhancement in acid mobility leading to image blur mediated by these factors.