Oxygen self-diffusion, D/sub o/x, and been monitored in single-crystal quartz in the temperature range 515°⁻⁻⁸⁵°sup 0/C under hydrothermal conditions. In the beta field, between 700° and 850°C, the data are represented by two linear Arrhenius relations for transport parallel and perpendicular to c. Values for Dâ (m² s⁻¹) are 2.09 x 10⁻¹¹ parallel to c and 3.16 x 10⁻¹° perpendicular to (1010). Values for ..delta..H (kJ mol⁻¹) are 138.54 parallel, to c and 203.72 perpendicular to (1010). At 700°C, in the total pressure range 11.5--100 MPa, D/sub o/x is independent of water (f(HâO)) and oxygen (f(Oâ)) fugacities between the Ni--NiO and FeâOâ--FeâOâ buffers. The results are consistent with diffusion via a simple charged vacancy mechanism under an extrinsic point defect regime. Further experiments are required to confirm the nature of the mobile oxygen defect.. A key aspect of the results is the observation that at the low water fugacities of the present experiments a hydrogen-containing defect appears to play no role in the oxygen transport mechanism. This is in contrast to other published sets of data and leads directly to the requirement for detailed interlaboratory comparisons.