The combination of differential electrochemical mass spectrometry (DEMS) and electrochemical quartz crystal microbalance (EQCM) is possible by means of a new dual thin layer cell. Thus, simultaneous real-time measurements of volatile products and deposited (or dissolved) species can be performed, which is demonstrated for CO-adsorption and desorption processes on Pt and a Pt-Ru alloy in sulfuric acid. In particular, it is shown that on a Pt-Ru alloy, electrode anions are adsorbed even at 0 mV vs. reversible hydrogen electrode.