A photochemical path for formation of volatile S and acidolytic paths for formation of volatile Si and/or C during exposure to ultraviolet light have been identified for two silicon-containing bilayer photoresists using atmospheric pressure ionization mass spectrometry (API MS) and secondary ion mass spectrometry (SIMS). Si and S are of particular concern because of their potential for irreversible adsorption onto nearby optical coatings inside a lithographic tool, causing permanent detuning. We describe the analytical techniques used and report estimates for absolute numbers of Si and S atoms lost from the resists. The data show that Si bound through oxygen is easily cleaved from the polymer by photogenerated acid while Si bound through carbon is not. Photodissociation of perfluorosulfonic acid from the photoacid generators results in significant loss of S. The data are compared to previous studies, and the advantages and limitations of these techniques for photoresist chemistry characterization are discussed.