Those EC and ECE type mechanisms are discussed in which the radical formed in the initial step of an electrochemical reaction (i.e. oxidation to a radical cation) and the products of the coupled chemical reaction reacts in a fast equilibrium type chemical reaction with another species. It will be shown, that the mathematical model of the complicated mechanism can be reduced to a simpler mathematical model, which is the formal model of the basic EC or ECE type reaction. An example is given for the application of the model for the case of electrodimerization with fast protonation at equilibrium.