Soft baking, one of the key steps in photoresist processing, is modeled theoretically. The relevant mechanisms involved in this step and the relative significance of each mechanism are discussed, and an analytical expression for the temporal variation of film thickness is derived. The applicability of the model derived is justified by fitting the experimental data far JSR IX150 and Shipley SPR510LA photoresists reported in the literature. We show that the rate of transfer of solvent is controlled by its convective transport near the film-gas interface.