In the context of a project to characterize and model the Alkali-Silica Reaction in concrete, the results of a series of tests on specimens with a single interface between aggregate and cement paste, and the comparison to the numerical results of a simplified diffusion-reaction Finite Element model are presented. The tests are conceived as the simplest test to characterize the basic constituent reactions of the process. The model implemented is a first operational version of a more general three-species diffusion-reaction model already formulated and in the process of implementation. In spite of its relative simplicity, the model described seems capable of reproducing the experimental observations with specimens tested under different diffusion boundary conditions, and provides a satisfactory basis for further development.