The oxygen isotopic composition of 76 samples of olivine in spinel-, garnet-and diamond-facies peridotites, and as syngenetic inclusions within diamond has been determined by laser fluorination (LF). δ 18 O V-SMOW values of olivine are almost invariant, averaging 5.18±0.28‰(2 SD) with an overall range of 4.8–5.5‰. Coexisting clinopyroxenes display positive Δ 18 O cpx-olivine fractionations averaging 0.4‰ that are consistent with isotopic equilibrium at normal mantle temperatures. The bulk compositions of spinel-, garnet-and diamond-facies mantle are similar, with a calculated bulk mantle δ 18 O of+ 5.5‰. Olivine inclusions in diamonds of Archaean age yield δ 18 O values that are indistinguishable from olivine in post-Archaean mantle and the LF data do not provide evidence for gross spatial or temporal δ 18 O variation in peridotitic mantle. There is no evidence that mantle hydration processes impart distinctive oxygen isotope characteristics, other than a subtle isotopic disequilibrium disturbing pyroxenes relative to olivine.