The use of polarography and linear-sweep voltammetry (LSV) for the study of a redox reaction O + ne o R when both O and R can be adsorbed (Langmuir isotherm) is examined, on the basis of a rigorous theoretical treatment presented earlier for a rotating disk electrode (r.d.e.) (E. Laviron, J. EIectroanal. Chem., 124 (1981) 19 and J. Electroanal. Chem., 140 (1982) 247). In aqueous medium on a mercury electrode, the reaction practically always occurs via the adsorbed species (surface redox reaction). However, two cases can be distinguished, according to whether the rate of desorption of the product of the reaction (in polarography) or of the adsorbed reactant (in LSV) is large or small when compared with the duration of the measurement (tau in polarography, RT/nFv in voltammetry). In the first case, the reaction appears as heterogeneous, with an apparent rate constant k(hm) which is much larger than the normal constant k(h), and which can be determined by using the classical theories for a heterogeneous reaction. In the second case, the reaction has a "surface" character, and the electrochemical surface rate constant k(s) can be determined by using the appropriate theories. The domain for each reaction can be represented by using adsorption diagrams log tau or logv vs. log(b(O)b(R))(1/2) (b(O),b(R); adsorption coefficients). The advantages of using polarography and cyclic voltammetry rather than r.d.e. voltammetry for the study of the above systems are discussed; they are theoretical (non-steady-state nature of the methods) as well as experimental (use of the dropping mercury electrode).