interface reactions, paradigms must be formulated to provide a framework for the interpretation of experimen­ tal data and for the construction of theoretical models. Phenomenological approaches have been based on a small number of rate equations for the concentrations or mole numbers of the various species involved in a par­ ticular system with the relevant rate constants either fitted (in the form of the Arrheniusparametrization) to experimental data or calculated on the basis of microscopic models. The former procedure can at best serve as a guide to the latter, and is, in most cases, confined to ruling out certain reaction pathways rather than to ascertaining a unique answer.