Results of chemical analyses of glasses produced in 46 melting experiments in air at 1,350[degree] C and 1,450[degree] C on rocks ranging in composition from nephelinite to rhyolite have been combined with other published data to obtain an empirical equation relating in (X liq Fe 2 O 3 /X liq FeO ) to T, lnf O 2 and bulk composition. The whole set of experimental data range over 1,200–1,450[degree] C and oxygen fugacities of 10?9.00 to 10?0.69 bars, respectively. The standard errors of temperature and log 10 f O 2 predictions from this equation are 52[degree] C and 0.5 units, respectively, for 186 experiments.