The method of single pixel exposures is applied for the determination of acid diffusion effects in negative chemically amplified resists. The wide range of crosslink density values contained in a single dot is used to determine nonlinear diffusion parameters. A reaction-diffusion model is developed where the diffusion coefficient D is a function of the crosslink density Theta. This function D(Theta) is evaluated for a given range of postexposure bake parameters in each case and the information obtained is used for proximity correction, also using the e-beam lithography simulation tool LITHOS. In order to test the model under different circumstances, two resists are studied, namely, the commercially available SAL-601 and the experimental epoxy novolac resist EPR. The diffusion coefficient is evaluated for each resist under the best processing conditions. The proximity correction procedure is fully demonstrated in the case of SAL-601.