Experimental data are given which provide a thermodynamic basis for predicting and understanding the distribution of Fe between metal, olivine and pyroxene as a function of both pO 2 and T. The composition of the run products was determined first by X-ray and refractive index measurements and then by electron microprobe analysis. From the experimental results, an expression is derived which relates pO 2, T and olivine composition: 1 2 log pO 2 = log N(FeSi 0.5O 2) − 14,540/T + 3.6 . The distribution coefficient ( K D ) of Fe and Mg between olivine and pyroxene according to the relation K D = N(FeSi 0.5O 2)N(MgSiO 3) N(MgSi 0.5O 2)N(FeSiO 3 is found to be independent of temperature over the range 1100–1300°C. However, K D is dependent on composition at these temperatures. Comparison of the experimental results with microprobe analysis of chondritic material indicates that the meteorites have consistently larger K D values than those determined by experiment. Inasmuch as a slight temperature dependence cannot be ruled out, this comparison shows that most, possibly all, chondrites either have equilibrated at temperatures below 1100°C or have not equilibrated at all. Carbon and hydrogen reduction processes are examined quantitatively. Several lines of evidence suggest that the bulk oxidation state was established at the time of accumulation. Large scale carbon or hydrogen reduction after consolidation cannot produce the observed degree of reduction. The bulk oxidation state must have been determined sometime prior to accretion while the dispersed material was in contact with a hydrogen rich environment.