Differential electrochemical mass spectrometry was employed for the investigation of the electrocatalytic activity of Ru deposits and of bimetallic Pt/Ru co-deposits. Using these modified electrodes the oxidation of the residues of CO, HCOOH and CH3OH was studied by cyclic voltammetry. The corresponding mass intensity of CO2 (m/z = 44) during the adsorbate oxidation clearly shows an increasing mass signal for CO2 more than 200 mV more negative at Pt/Ru in comparison with that on pure Pt. In addition, the formation of methanol residues on the Pt/Ru surfaces was studied and compared with those on pure Pt and pure Ru. It is shown that the peak potentials for the formation of CH3OH residues at Pt/Ru surfaces are shifted by 70 mV in the negative direction compared with pure Pt, while pure Ru shows no formation of methanol residues. Nevertheless, on the Ru surfaces CO and HCOOH do form adsorbates. The results presented demonstrate that the bifunctional mechanism, proposed earlier for the methanol oxidation at Pt/Ru, must be modified.