The self-diffusion of 18O-enriched oxygen in forsterite was determined by measuring the bulk concentration using the nuclear reaction 18O( p,α)15N. Important technical improvements led to a more precise determination of the 18O diffusion profiles. These results indicate that the self-diffusion coefficient D* is independent of the annealing duration and the oxygen partial pressure. In addition, the D* values are found to be lower by nearly 1 order of magnitude than those previously published (preexponential factor D0~2.3¿10-6cm2/s, activation energy, ~70 kcal/mole).