The citrate concentration of electroless CoP plating solutions was found to decrease slowly with long-term use. The mechanism of citrate loss is most likely a thermal decarboxylation. In the electroless CoP solution, the decarboxylation reaction is catalyzed by the Co(II) ion and does not depend on any of the other solution components. The reaction occurs only in oxygen-containing solutions and its rate is limited by the rate of oxygen diffusion. The effect of other metal cations on citrate decomposition has been examined. Only those cations with a readily available 3+ oxidation state in aqueous solution seem to act as catalysts. © 1989 Chapman and Hall Ltd.