The FeMg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite (mg = 67) in the presence of quartz and water at pressures of 20-35 kb and temperatures of 800-1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InKD((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InKD and pressure in the above P.T range. This FeMg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe3+ in phengite must be made. If the Fe3+ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only.