Nickel maleate decomposed in vaccum 270–310°C without melting. Carbon dioxide and a residual, highly-divided, nickel + carbon mixture were the principal products; there were smaller amounts of carbon monoxide and certain hydrocarbons. There was an initial deceleratory rate process, identified as a surface reaction, during which some 6–8% of the gaseous products were evolved. This was immediately followed by a perceptible acceleratory period of a relatively rapid reaction, the rate of which thereafter progressively diminished. This reaction was satisfactorily described, 0.2<α<0.8, by the equation (1–α)^–⅔–0.91 =kt. This equation was derived assuming that the rate-controlling process was the continued first-order nucleation of product phase particles at the surfaces of small reactant crystallites, the size of which progressively diminished as reaction proceeded. Growth of such nuclei was inhibited by carbonaceous residue deposition and the constant (0.91) resulted from the occurrence of the initial reaction. The activation energy measured (188 ± 12 KJ mol^–1) and the frequency factor estimated for the nucleation step(1.35×10²⁹ molecules m^–2s) were compared with values for other nickel carboxylates.