We have investigated the suppression of acid diffusion during postexposure baking (FEB) of chemical amplification resists in terms of the molecular control of base matrix polymers to control acid-diffusion channels. According to a previously proposed model, acid diffuses along acid-diffusion channels in chemical amplification resists. The acid-diffusion channels are composed of OH groups outside the base matrix polymers (active OH groups) and vacancies in the matrices. The negative-type chemical amplification resists used in this work consisted of cresol novolak with controlled molecular-weight distributions as the base matrix polymers, an acid-catalyzed crosslinker of melamine resin, and acid generators of onium salt. Measurements of the dependence of the pattern sizes of isolated lines on the FEB time made it clear that acid diffusion defines the resist pattern sizes based on Fick’s law. The densities of the active OH groups play a critical role in acid diffusion, and a base matrix polymer with fewer active OH groups is expected to result in less acid diffusion. Therefore, we can suppress acid diffusion by reducing the number of oligomers in the base matrix polymers by a precipitation method, since these oligomers stereochemically include many active OH groups. We have obtained high resolution by using a resist with an oligomer-free resin. The acid-diffusion model was also confirmed based on infrared absorption spectroscopy and thermal analysis. We conclude that acid diffusion can be suppressed by controlling the acid-diffusion channels.