A treatment of the ac faradaic impedance of a cell is presented for a redox system with adsorption of the reactants; the adsorption, assumed to be fast (at equilibrium), obeys a Langmuir isotherm. The variations of the phase angle phi are analysed. When the adsorption coefficients are equal to zero, phi is characteristic of a heterogeneous reaction; when they become large enough, phi corresponds to a purely surface process. For intermediate values, an apparent heterogeneous reaction occurs, whose rate constant is larger than that of the true heterogeneous process. The case of other isotherms is considered. The Nyquist diagrams are presented. Earlier treatments of the problem are discussed.