The assemblage quartz–K feldspar–biotite–garnet–sillimanite is common in high-grade pelitic rocks. Diverse compositions of biotite and garnet from this assemblage reflect differences in Ptotal, , and T. For rocks from a small area, Ptotal probably was nearly constant. In order to show the separate effects of and of T on mineral composition, a plot of conditions indicated by various rocks with this mineral assemblage on a –T diagram is presented. This is accomplished by treating the minerals and the fluid phase as ideal solutions, by assigning reasonable –T values to one rock, and by assuming entropies for two dehydration reactions. Rocks from the Thor-Odin gneiss dome in British Columbia reflect a temperature range of about 200 °C and relatively low water pressures.