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). delta(18)O(V-SMOW) values of olivine are almost invariant, averaging 5.18 +/- 0.28 parts per thousand (2 S.D.) with an overall range of 4.8-5.5 parts per thousand. Coexisting clinopyroxenes display positive Delta(18)O(cpx-olivine) fractionations averaging 0.4 parts per thousand 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 delta(18)O of +5.5 parts per thousand. Olivine inclusions in diamonds of Archaean age yield delta(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 delta(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.