The anodic oxidation of formaldehyde on gold in alkaline aqueous solutions was studied by electrochemical impedance spectroscopy. One equivalent circuit was found to mimic all impedance spectra measured for normal formaldehyde and deuterated formaldehyde (CD2O) in various alkaline solutions at various voltages. The resistors and capacitors constituting the circuit depend logarithmically on the voltage applied, and the circuit can be explained in terms of reaction steps proposed in the literature. A negative capacitance (inductive loop) was encountered at low frequencies, which is explained by a rate-determining C-H bond rupture step in the overall kinetics of the reaction, as it was more apparent for solutions containing CD2O instead of CH2O. Data were Kramers-Kronig transformable and a chi(2)-value of 10(-4) or less was attained in all fits. At least four time constants were revealed by the impedance spectra as a result of positive capacitive, negative capacitive, resistive, and diffusive effects.