Viscosities of several compositions in the albite-anorthite system (Ab80An20, Ab50An50, and Ab25An75) were measured over a wide range of temperature (800¿--1600¿C.) The log &eegr; versus 1/T relations for all compositions show continuous curvature. In the high-temperature region (T>1200¿C) the viscosity increases monotonically with increasing Ab content, but the change of viscosity with temperature increases with An concentration. Hence the viscosity-temperature relations show a region of crossover, and at sufficiently low temperatures (T≲750¿C) the viscosity increasing An content. The predictions of models based on free volume theory and the semiemperical models of Bottinga and Weill (1972) and Shaw (1972) are in close accord with the data (within ¿0.2 in log10&eegr; in nearly all cases).