Calculation of mass transfer to and from electrodes when homogeneous reactions occur in the mass-transfer boundary layer often requires numerical solution because the transport equations are generally nonlinear. Although finite-difference or finite-element methods are common, they may sometimes fail if the model equations are too stiff, for example, when a thin reaction zone is established by fast reactions. The structure of such problems is examined analytically by a perturbation analysis of a simplified model system. Numerical calculations of aqueous carbonate-species concentration profiles and interfacial pH are done for the boundary layer on a cathode where hydroxide ion is produced by the reduction of oxygen and reacts rapidly with the carbonates. These examples demonstrate how such problems may be formulated when very fast homogeneous reversible reactions are involved. (c) 2007 The Electrochemical Society.