The diffusion of p-toluene sulfonic acid (PTSA), a photoacid, in novolac was investigated at temperatures below and just above the glass transition temperature (T-g=91 degrees C) using a technique based on ellipsometry. Bilayer samples consisting of a blended film of novolac and PTSA and a film of pure novolac were held at a constant temperature for a known period of time and then quenched. The diffusion of PTSA from the supply layer into the initially pure novolac transforms the initial step function profile in refractive index to a more diffuse gradient in refractive index. Ellipsometry data from the samples could be analyzed to obtain diffusion coefficients using a Fickian model of diffusion and, the experimentally determined dependence of the index of refraction as a function of PTSA concentration. The values of the diffusion coefficients were 10(-14) to 10(-12)cm(2)/s for temperatures between 50 and 95 degrees C. The activation energy for diffusion below the glass transition temperature was 67 kJ/mol. At 95 degrees C, just above T-g of novolac, the diffusion coefficient and activation energy increased significantly. This simple and convenient experimental technique provides useful data for the optimization of resist processing.