The influence of source material, decomposition and annealing temperatures on the reactivity of nickel oxide surfaces has been studied. The process of thermal decomposition in air of four nickel salts (acetate, oxalate, carbonate and hydroxide) was followed using thermogravimetric and differential thermal analysis, differential scanning calorimetry, B.E.T. surface area determination and infrared spectroscopy. Nickel acetate and oxalate gave products consisting of nickel metal and nickel oxide. The reduction mechanisms are described. The carbonate and hydroxide gave only nickel oxide, but the martensitic decomposition of the carbonate (ΔH_t= 28.8 kJ mol^–1) is considerably more energetic than that of the hydroxide (ΔH_t= 2.8 kJ mol^–1). The loss of surface area in the decomposition is correspondingly larger for the carbonate. Infrared studies of the degassing process from 25–500 °C in vacuo produced evidence for reduction on the ex-carbonate NiO surface which correlated with reactivity in dissolution kinetics studies. An absorption at 2193 cm^–1 was observed, with maximum intensity after 300 °C decomposition, assigned to an Ni⋯(O—C—O)^– species resulting from CO interaction with surface O^–ions. Other carbonate and hydroxyl surface groups were characterised for each oxide at different decomposition temperatures.