The theory of a surface electrochemical reaction (strong adsorption) occurring in two 1e(-) steps O reversible arrow R reversible arrow X, when R can undergo a dimerization R --> D and X a first-order reaction X --> Z is presented for polarography. It is assumed that adsorption obeys a Langmuir isotherm. The equation of the half-wave potentials as functions of the diverse parameters (rate constants, concentration, surface concentration, drop time) are given; the effects of the two chemical reactions are additive. Kinetic diagrams are constructed. The results can be used to diagnose the type of reaction and to calculate the chemical rate constants. The effects of the dimerization on the half-wave potentials of the two stages do not depend on their separation; in the case of a global 2e(-) step, they remain hidden until the rate becomes large enough to cause a splitting into two 1e(-) waves. These results are generalized to the case where the electrochemical reactions are heterogeneous, and the chemical reactions homogeneous.