A simple model of the galvanostatic oscillations that occur during the oxidation of hydrogen at platinum electrodes in the presence of electrosorbing metal ions and specifically adsorbing anions is presented. It consists of three ordinary differential equations describing the degree of coverage of the electrode with underpotential-deposited metal and with specifically adsorbed anions; the third variable is the potential drop across the double layer. This model is able to reproduce the potentiostatic experiments of the system. The galvanostatic behavior is examined by integration of the equations and by bifurcation analysis. The experimental dependence on the control parameters is reproduced in all main features. The parameters were altered to check the model for robustness and to determine the essential processes leading to oscillations. Mixed-mode and period-doubled oscillations were also found. Inclusion of a fourth variable, the concentration of metal ions in front of the electrode, leads to a better quantitative description of oscillations at low metal ion concentrations.