The perpetual advancement of materials and equipment for microlithography has resulted in the ability to print critical dimensions that approach the size of the molecules that make up photoresists. As a result, molecular scale effects such as line edge roughness have become a concern for both resist manufacturers and process engineers. In this work we have investigated the increasing importance of molecular level effects, especially in terms of the contributions of the exposure and postexposure bake (PEB) steps to spatial variations in film composition. A mesoscale simulation of the PEB was used to model the discrete mass transport and reaction events that create the changes in film composition responsible for resist function. Local irregularities in resist composition are generated during the PEB, the magnitude of which can be related to the local concentration of acid. This study is focused on the establishment of an understanding of the effects of process and composition variables on the reaction product distribution. The reaction product distribution was calculated for an APEX(R)-like resist under a variety of exposure and bake conditions. These process variables have a profound influence on spatial irregularities in the composition gradient. Ultimately, it is the interaction of this reaction product distribution with the development process that will determine line edge roughness.