The mechanism of the post exposure bake effect, PEB, was studied in detail on several model photoresist compositions containing well defined PAC structures and novolak resins. Under standard processing conditions, reduction of standing waves was found to be dependent on the PAC functionality, PAC molecular size, resin glass transition temperature and on the solvent retained in the photoresist film. These results are all consistent with diffusion as the dominant mechanism of the PEB effect. PAC size was varied from 366 to over 1800 Daltons and resins were selected with glass transition temperatures, Tg, below and above the PEB temperature. Above the resin Tg, the PEB effect was found to be strongly relates to PAC size, with very large PAC molecules showing miniml standing wave reduction. When the PEB temperature is below the resin Tg, standing waves are eliminated only in photoresists containing the smaller PACs. Solvent retention enhances the PEB effect. PAC thermal decomposition, with or without indenecarboxylic acids, was found to be an insignificant contributor to standing wave reduction in normal PEB processing. Furthermore, experimental and simulation results indicate that the initial size of standing waves depends on the PAC functionality and the dissolution selectivity of the resist system. The dependence of diffusion on PAC size has important implications for lithographic modeling, particularly for resist compositions containing mixtures of PAC's of different sizes and degree of esterification. Resist compositions with very large PAC's or very high Tg resins may show limitations with respect to the range of practical PEB processes.