The electrocatalytic properties of a fuel cell catalyst ink in thin film form based on Vulcan XC72/Pt (30%) and Nafion was investigated by means of a flow cell of impinging jet type in flows of H2 and H2+CO dissolved in H2SO4 electrolyte. CO ad layers of different coverages were produced by a dosing procedure and their electrooxidative removal was studied. Ru was deposited onto the ink in situ by electroredn. and the basic voltammetry of such electrodes was investigated as well as the influence on CO ad layers and CO-tolerance. A Nafion film on top of the ink film influenced both the behavior of CO ad layers and CO-tolerance. The feasibility of the flow cell for investigation and modification of ink electrodes were demonstrated.