A sharp interface model has been developed for non-catalytic liquid-solid reversible reactions. It is shown that a sharp reaction interface can occur, in certain circumstances, even if the reaction is reversible. Because the pseudo-steady state approximation may not be valid for liquid-solid reactions, this assumption was not made in the model. This led to the need for numerical resolution and the model was solved using orthogonal collocation both in the unreacted con and in the ash layer. Owing to the equilibrium relationship, which holds at the reaction interface, the concentrations of the liquid species are coupled at this location, resulting in a possibly non-linear set of algebraic equations that must be solved in order to evaluate concentrations at the boundaries. An analysis is made of the influence of the parameters that characterize the model. The effect of reversibility is a decrease in the driving force for diffusion.