This paper addresses the general problem of multicomponent transport and heterogeneous reaction in Lévêque's approximation. The problem always admits an analytic solution when the surface reactions are first order in the concentrations. Here, explicit expressions for the concentration distributions are found for the case of a single reversible surface step, but the procedure equally applies to arbitrarily complex sequences of such steps. The behavior of the solution depends mainly on the value of a dimensionless axial parameter. Aside from axial distance and mean velocity, it involves all diffusion coefficients, reaction rate constants and stoichiometric coefficients. For small values of this parameter the solution behaves as if the surface were a source of constant flux; for large values, as if the surface were at constant concentration. Thus the solution exhibits a continuous change from a Neumann to a Dirichlet problem. © 1978.