Samples prepared from synthetic single crystals of olivine have electrical conductivity that differs significantly from samples prepared, under similar conditions, from San Carlos olivine. In particular, the oxygen fugacity with which the sample was treated before being loaded into the laser-heated diamond anvil cell (LHDAC) has a large effect on the electrical conductivity of magnesiow¿stite/perovskite assemblages synthesized from San Carlos olivine and no significant effect on those synthesized from the synthetic olivines. This effect is likely due to the presence of nickel in San Carlos olivine. However, because the main effect of nickel in olivine is to diminish the extent of the redox stability field of olivine, this points to a possible effect of the internal oxygen fugacity of the LHDAC on the assemblages produced during transformation. Electron microscopy examination of samples transformed to magnesiow¿stite/perovskite assemblages in the LHDAC under identical conditions detected substantially more metallic iron in assemblages produced from San Carlos olivine. Analysis of the experimental procedure employed for transformation of silicates in the LHDAC indicates that oxygen from air trapped in the porous powder during pressurization can provide a condition of constant oxygen fugacity in the material. In the temperature gradient that exists during laser heating, a condition of constant oxygen fugacity can lead to very complicated phase assemblages, varying from highly oxidized to highly reduced, at least for materials containing transition metals. ¿ 1997 American Geophysical Union