From a large number of experimental data on olivine-liquid and pyroxene-liquid equilibria, it is revealed that there is a linear relationship between the mole percent of MgO in olivine and/or pyroxene saturated liquid, temperature and pressure. The effect of dissolved water content on the linear relationship has also been taken into account based on recently reported experimental data in the hydrous systems. The linear equation allows one to calculate the temperature from the MgO content and pressure or the MgO in the liquid from the temperature and pressure. The standard deviations of calculated temperature and MgO content of the anhydrous liquid are about ¿30 ¿C and within 3 mol %, respectively. The standard deviation of the temperature is the same or less than those of previously reported geothermometers. This geothermometer has the advantage that it is unnecessary to know the FeO content of the liquid or oxygen fugacity. ¿ 2000 American Geophysical Union