In reactions where product selection involves competition between dimerization and first-order steps, selectivity may change on going from direct electrolysis to redox-catalyzed electrolysis. The variation of product distribution with pertinent competition parameters is rigorously and quantitatively established in each case. The physical reasons underlying this prediction are discussed. They are shown to derive from the development of a space-dependent concentration profile of the product selection intermediate. The particular form that this profile takes in each case is a function of the nature and rate of the step that produces the key intermediate and of the rates of the follow-up steps.