Summary: Microprobe analyses of eight ilmenites, a titaniferous magnetite, and a rutile are presented. They were synthesized at pressures from 5 to 26 kb and temperatures between 1075 and 1225 °C in the anhydrous melting interval of a tholeiitic andesite from the Snake River Plain, Idaho, U.S.A. Both the Fe/Ti distribution between coexisting ilmenite and titanomagnetite at 11 kb/1075 °C and the low calculated Fe³⁺ contents of the other ilmenites confirm previous suggestions that the f_o2 in these experiments was buffered by graphite capsules to values near those generated by the wüstitemagnetite assemblage. Mn in the ilmenites (0·30–0·51% MnO) shows moderate negative correlations with both the pressures and temperatures of their formation, whilst AI (0·41–1·06 % Al₂O₃) shows poor positive correlations with these parameters. In contrast, the only significant trend shown by Mg in the ilmenites (2·09–5·26 % MgO) is with the position of each experimental run in the melting interval of the lava. Mg/(Mg + Fe²⁺) of the ilmenite decreases during equilibrium crystallization at a given pressure and appears to be controlled solely by Mg/Fe²⁺ distribution amongst the coexisting ferromagnesian minerals and interstitial liquid (glass).