A new method was developed to study the diffusion of photogenerated acid in chemical amplification resist systems which allowed an estimation of the diffusion range by simple means. The acid mobility was investigated for two different resist systems under various process conditions. It was found that solvent traces in the film cause a very strong increase of the acid mobility. In order to control the diffusion range, the post‐exposure‐bake temperature must be below the glass transition temperature. For one resist system, the increase in resist sensitivity with increasing baking temperature was much smaller than the corresponding increase in diffusion range. The results corresponded well with those obtained by lithography with the same resist.