Sub-100 nm lithography poses strict requirements on photoresist material properties and processing conditions to achieve necessary critical dimension control of patterned structures. As resist thickness and feature linewidth decrease, fundamental materials properties of the confined resist polymer can deviate from bulk values and impact important processing parameters such as the postexposure bake (PEB) temperature. The effects of these confinement-induced deviations on image or linewidth spread have not been explored. In this work, we characterize the resist thickness dependence of the spatial extent of the reaction-diffusion process in a chemically amplified photoresist system under varying processing conditions. Bilayer samples are prepared with a lower layer of a protected polymer (p-tert-butoxycarboxystyrene) and a top layer of a de-protected polymer [poly (4-hydroxystyrene)] loaded with a photoacid generator. After flood exposure, PEB, and development, changes in the thickness of the protected polymer provide a measure of the spatial extent of the reaction front between the polymer layers. The velocity of the reaction front is significantly reduced with decreasing thickness of the protected polymer layer under identical processing conditions.