By simultaneously measuring the copper deposition rate and the current, we have determined the partial currents corresponding to the oxidative and reductive half-reactions for electroless Cu deposition from an EDTA, formaldehyde solution. These experiments were performed in a solution containing both copper and formaldehyde. The copper deposition rate was measured in situ with a quartz crystal microbalance. The resulting partial I/V curves are different from those obtained by others who used separated (one component missing) half-cells. Specifically, we find that formaldehyde catalyzes the electrochemical reduction of Cu(EDTA). These observations help explain discrepancies between previously measured I/V curves for the independent oxidative or reductive half-cells, and the predictions of mixed potential theory concerning the electroless Cu deposition rate. © 1989, The Electrochemical Society, Inc. All rights reserved.