Micromolding in capillaries is a soft lithography method for patterning materials. We have studied the diffusion of solvent from the excavated microsized channels in the stamp into the PDMS material, both theoretically and experimentally. It was demonstrated that a model of 1-D diffusion of solvent through a PDMS stamp, coupled with a mass conservation of the solvent in the channels, leads to a quantitatively accurate model for the velocity of the boundary between liquid-filled and vapor-filled microchannels in the stamp. With the model the diffusion coefficient of the solvent into PDMS was successfully determined.