Decades of study have documented several orders of magnitude variation in the oxygen fugacity (ƒO2) of terrestrial magmas and of mantle peridotites. This variability has been primarily attributed either to differences in the redox states of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). I will show, however, that the well-understood phase equilibria of fertile peridotite that produce the plagioclase->spinel->garnet lherzolite transitions and the variations in Al content in pyroxenes within each of these facies can lead to significant, systematic variations in ƒO2 in the upper ~120 km of the mantle. In addition, although isobaric melting generally leads to decreasing ƒO2, isentropic decompression melting can result in the ∆FMQ of peridotite increasing by ~1 log unit. This also reflects primarily the effects of solid–solid phase transitions, with melting itself only introducing a small perturbation on melt-absent trends. These effects of pressure- and temperature-dependent phase equilibria on the ƒO2 of peridotite of constant composition and on lower degree partial melts of peridotite are likely to be superimposed on variations due to bulk O2 content: As a result, these effects should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.