The active surface area of a Pd-Si alloy (80-20 at.%) was determined by means of the anodic charge associated to the desorption peaks of underpotential deposited hydrogen atoms. The value found was very close to that expected for a complete monolayer of Pd-H, considering the percentage of Pd on the surface. The results are comparable to those obtained by the classical cyclic voltammetric method of reduction of oxides grown at different inversion potentials. The agreement suggests that one full monolayer of adsorbed hydrogen is formed on the Pd portion of the alloy surface.