Equilibrium oxygen isotopic fractionation factors between coexisting olivine and enstatite at mantle temperatures and pressures, 1000-1400°C and ≥ 13 kbar, were determined from a series of silicate-witherite oxygen isotope exchange experiments. Silicate-witherite fractionations ranged from - 2% 0 at 1000°C to - 1% 0 at 1400°C, and showed a systematic jump in magnitude at 1200°C of ~0.5% 0 which remains to be explained. Enstatite-witherite fractionations varied nonlinearly with respect to inverse temperature squared. The high-temperature pyroxene-witherite data suggest enstatite undergoes a reversible structural change as it approaches an incongruent breakdown reaction to olivine, quartz, and sanbornite, which may account for the observed nonlinearity. A least-squares fit to the low-temperature equilibrium enstatite-olivine fractionation data forced through the origin has the functional form 1000 In αen- olT( K) = (1.41 ± 0.43) × 10 6/ T2-(0.0 ± 0.24) for temperatures greater than 1000°C ( ± 2σ errors). At equilibrium under mantle conditions, the oxygen isotopic composition of enstatite is always heavier than, or equal to, that of coexisting olivine.