State-of-the-art microlithographic processes used to make features smaller than 0.25 microns are based upon deep-UV lithography and chemically amplified resists (CARs). In these resists, photoacid generated during exposure initiates cascading deprotection reactions during post exposure bake (PEB) to form a developable image. Reaction may not be limited to the illuminated areas since the photo-generated protons may diffuse outside this region; therefore, it is important to understand the diffusional characteristics of the photoacid. In this contribution, macroscopic free volume changes in the photoresist film were studied using multi-wavelength interferometry, and acid mobility and concentration during PEB were studied using crystal violet. This probe was added to the Shipley Apex-E 2408 DUV photoresist, and measurements were carried out in situ on quartz substrates. Crystal violet is a triphenylmethane 'propeller' molecule whose excited-state lifetime increases as the local free volume decreases, and molecular rotation is hindered. This feature can be used to characterize the free volume in the photoresist using ground- state recovery experiments. In addition, crystal violet has three protolytic forms, each with a unique absorption spectrum, and a calibration of the absorption spectrum as a function of acid concentration was used to measure the acid concentration during PEB. These studies illustrate the utility of spectroscopic techniques to characterize CARs in situ.