The copper complex with a single 1,10-phenanthroline ligand can be irreversibly adsorbed on graphite electrodes, catalyzing the reduction of both O-2 and H2O2. The electrocatalytic kinetics of both substrate reductions were studied by cyclic voltammetric and rotating disk electrode methods. The addition of a very small quantity of a species such as SCN-, H2S or HCN in the test solution poisons the electrocatalytic activity for O-2 and H2O2 reduction. A theoretical model is proposed to describe this poisoning effect based on the coordination equilibrium between poisoning species and surface adsorbed catalyst, the inner-sphere mechanism of substrate reduction, and Koutecky-Levich theory. The model is supported by experimental results. The surface behaviour of the adsorbed [Cu(phen)](2+)(ads) in the presence of the poison species clearly shows the formation of a new surface complex with [Cu(phen)](2+)(ads). These new "poisoned" surface complexes are electrocatalytically inactive towards O-2 and H2O2 reduction. The possible application of this poisoning effect for the analysis of trace SCN-, H2S and HCN was explored.