The influence of H/D substitution in both formaldehyde and water on the rate of formaldehyde oxidation at copper was studied by means of differential electrochemical mass spectrometry and a thin-layer flow-through electrochemical cell as a function of the electrode potential from -0.7 to -0.4 V (SHE) at pH 13 and 25-degrees-C. k(H)/k(D) almost-equal-to 1 was found for the anodic oxidation of formaldehyde when H2O is replaced by D2O. It was determined that k(H)/k(D) depends on the electrode potential for H/D substitution in formaldehyde and reaches the highest value of ca. 7 at low potentials and drops to ca. 2.5 at higher positive potentials. No isotope effect was detected in the course of catalytic formaldehyde dehydrogenation under open-circuit conditions for H/D substitution in both formaldehyde and water.