At a modified electrode, electrocatalysis is accomplished by an immobilized redox substance acting as an electron transfer mediator between the electrode and a reaction substrate. Such mediated electrocatalysis is possible with monomolecular and multimolecular layers of the redox substance. The electron transfer mediation can assume several special forms; these are identified and experimental examples are given. The differences between electrocatalytic behaviour of monomolecular and multimolecular layers are discussed; electrocatalysis in the latter circumstance can include reaction rate elements of electrochemical charge and substrate migration through the multilayer in addition to the chemical rate. Theoretical ideas are presented that interconnect these three rate elements, to show that either all of the multilayer sites can participate in the electrocatalytic reaction, or only about the equivalent of a monolayer, depending on the relative rates of the electrochemical charge transport, the diffusion of substrate, and the chemical reaction rate.