The activity of end member I, a, in a solid solution can be defined as a, = a con, y, where y, is an activity coefficient describing the departure from ideal solution and a,con is the ideal contribution to activity. Statistical thermodynamics yields an explicit expression for a,con in terms of configurational entropy and mixing on sites. This general expression applies to binary, ternary, and complex multicomponent solid solutions. Applications of this expression to feldspars yields an adjustment of the two-feldspar geothermometer. Data in the system albite-orthoclase-anorthite-h2o fit well enough to suggest that the proposed geothermometer is accurate to +/- 50 deg. C in the range of 650 deg. to 900 deg. C for coexisting plagioclase and alkali feldspar for which 0.15 < Xaf/ab < 0.50.