Decomposition of nickel formate, nickel malonate, nickel maleate and nickel fumarate in oxygen yields nickel oxide and carbon dioxide only. The shape of each fractional reaction-time curve is comparable with that for the vacuum pyrolysis of the salt, there is no evidence that the product oxide provided a progressively increasing barrier to the progress of the oxidation process. The values of the activation energies for the decomposition reactions of all four salts are 150 ± 10 kJ mol^–1 and different from those for reactions in vacuum. From the observations we conclude that the geometry of advance of the reactant/product interface, during nucleation and subsequent growth of the product phase, for reactions in oxygen and in vacuum are closely comparable, if not identical. The chemical processes occurring at the metal carboxylate/nickel oxide interface during the reaction in oxygen are discussed. It is suggested that the rate controlling process is largely determined by the surface and defect properties of the nickel oxide product phase.