Acid diffusion plays an important role in chemically amplified resists. Acids are generated from acid generators upon deep ultraviolet Light (DUV) irradiation. Physically, what the DUV illumination creates is an electrolyte solution in a nonequilibrium state. In addition to the force due to the concentration gradient, a charged particle undergoing diffusion motion experiences a dielectric friction force from the medium and the Coulomb force created by the surrounding charged particles. The Coulomb force between charged particles can be very important in a medium with a low dielectric constant. In such a case, the diffusivity of the counter ion can have a great impact on the "effective diffusivity＇＇ of the acid. The diffusion of acid and its counter ion are correlated strongly. Besides the Coulomb interaction, the acid-base chemical equilibrium also affects acid diffusion. In this article, a simple model of acid diffusion including the effects of acid-base equilibrium and Coulomb interaction is proposed. The theory proposed here shows that the "effective" acid diffusivity is mainly determined by the diffusivity of its counter ion, which can account for the acid generator size effect on acid diffusion observed in experiments.