In multi-component chemical systems, thermodynamic fluxes linearly depend on thermodynamic forces through the matrix of Onsager’s coefficients. By applying Onsager’s relationship to the PEB diffusion of acid and other relevant chemical components, we deducted a generalization of the traditional PEB diffusion-reaction equations. This generalized system includes cross-diffusion coefficients. We have shown that certain combinations of these coefficients predict acid propagation with constant speed, even when all coefficients are constant. This serves as an alternate explanation to the conventional theory, which has to assume exponential dependency of the acid diffusion coefficient on activated site concentration. The model with cross-diffusion coefficients has additional degrees of freedom, which makes it more flexible in calibrations to the empirical data.