Postexposure bake (FEB) dependence of photogenerated acid diffusion was investigated in a chemically amplified deep ultraviolet positive resist. The resist consisted of a tert-butoxycarbonyl protected polystyrene as base resin and 2,4-dimethylbenzenesulfonic acid derivative as photoacid generator. The diffusion length of photoacid increased with increasing FEB temperature or its time. Moreover, the activation energy of acid diffusion reaction within the resist film became smaller, with increased exposure dose. It is considered that hydrophilic OH sites of the base resin generated by the deprotection of hydrophobic protecting groups has a role as one of the diffusion paths in the polymer matrix. Furthermore, it was found that the diffusion coefficient under high FEB conditions was affected by the acid reduction. Based on the analysis of diffusion characteristics, clear correlation between acid diffusion in the resist film and FEB conditions was obtained. These results are useful for improving both resolution capability and pattern profiles.