The oxidation of cubic diamond plates was experimentally investigated at 100 kPa, at 1000, 1050 and 1100 °C and at variable oxygen fugacity (fO2) to evaluate the effect of fO2 on diamond preservation and to examine the development of secondary diamond morphology. The diamond oxidation rate was determined from weight and size changes of the diamonds and shows a similar fO2-dependence at three temperatures (T): Ln V = 0.57 log fO2 + Tterm, where V is diamond oxidation rate and Tterm is the term dependent on temperature. The derived relationship can be used to evaluate the significance of the earlier established variations in the redox state in several EKATI mine property kimberlites, N.W.T., Canada. The surfaces of etched diamonds from the experiments were studied using high-resolution field emission scanning electron microscopy. At 100 kPa the shape, orientation and size of the etch pits have no consistent correlation with temperature, fO2 and the reaction rates. Increase in the CO2-content of the reacting gas results in an evolution from irregular cavities to pits formed by the development of (100) faces and further to more regular forms with (111) faces. These results agree with high pressure data and suggest that the features developed on oxidized diamonds are determined by variations in the reaction rates in different directions of the diamond lattice, which are dictated by the molecular composition of the reacting fluid (or gas). A high CO2 content favours fast oxidation rates in the  direction.