The effect of the CN-ion on the formaldehyde electro-oxidation reaction has been examined in basic EDTA solutions at 75°C on Au, Pd, and Cu electrodes. The results are compared with the cyclic voltammetry of these three metals in HCHO-free EDTA solutions. The cyanide ion poisons HCHO electro-oxidation on Au and Pd; on Cu, however, substantially higher HCHO-oxidation currents can be sustained in the presence of cyanide. The poisoning on Au and Pd is related to the suppression of dissolution of the metal by the formation of a sparingly soluble metal cyanide on the surface. Metal dissolution is apparently also suppressed on Cu electrodes, but the greatest promoting effect of CN-occurs in a potential range in which CN-adsorbs on the unoxidized Cu surface. © 1984, The Electrochemical Society, Inc. All rights reserved.