With the shrinkage of pattern sizes, the elucidation of reaction mechanisms at molecular level has become essential to resist design. Especially, proton dynamics is the most important issue for sensitivity and resolution of chemically amplified resists. Also, nanoscale topography of patterned resist surface such as line edge roughness may be explained by precise proton dynamics. In chemically amplified resists for post-optical lithographies such as EUV and electron beam lithography, it has been reported that protons come not from acid generators but from base polymers. Determining proton sources is a key to understanding reaction mechanisms at molecular level. In this article, we investigated deprotonation mechanism of poly(4-hydroxystyrene) and poly(4-methoxystyrene) on the exposure to ionizing radiation.