Diamond oxidation experiments were undertaken in a piston-cylinder apparatus at 1150 to 1500 °C and 1 GPa to understand the mechanism of diamond oxidation in kimberlite melts and to determine the main rate-controlling parameters for this process. Only surface graphitization, and no diamond resorption, occurs in melts that are fluid undersaturated (synthetic kimberlite, carbonate melt, alkaline basalt, CaO-MgO-SiO2-H2O-CO2 melts). In contrast, fluid oversaturated conditions (as evidenced by the presence of bubbles) produce resorption features commonly seen in natural diamonds recovered from kimberlites. The diamond oxidation rate is the same in the melts with a free fluid phase, in a pure H2O or CO2 fluid, suggesting that the process of diamond oxidation is its reaction with the fluid and not with the melt. Both CO2 and H2O oxidize diamonds at a similar rate, but produce very different surface features. Therefore, the surface features of natural kimberlite-hosted diamonds may provide information on the relative proportion of H2O and CO2 in the kimberlitic fluid. The common diamond morphologies imply significant amount of H2O. The absence of diamonds with surface graphitization and the abundance of resorbed diamonds in kimberlites suggest the presence of a free fluid phase in kimberlite magmas for hours or days. We found no correlation between the rate and character of diamond oxidation and the physical properties of diamonds (nitrogen content, color).