The MgO content of komatiite lavas is an important measure of their formation temperature deep in the Archaean mantle, and forms the basis for models of the early Earth’s thermal and chemical evolution(1-5). Estimates of the primary MgO content of komatiites are sensitive to the oxidation state-characterized by the oxygen fugacity (f(O2))-assumed for the magmas during their crystallization. Despite two decades of study, however, f(O2) is still poorly constrained for these lavas. Here I present an estimate of the f(O2) for komatiite flows, based on the systematics of vanadium partitioning between komatiitic liquid and olivine in six well-characterized komatiite flows of varying ages. This approach shows that the oxidation state of several of these Archaean lava flows was the same as, or possibly more oxidizing than, that of present-day oceanic basalts. These results may require a downward revision of the mantle melting temperature estimated for many komatiites by about 50 degrees C, and suggest that the mantle was unlikely to be much less oxidized during the Archaean era than at present.