A closed-system Sr isotope exchange model involving isotopic equilibration among minerals by volume diffusion within grains and grain-boundary diffusion between grains is used to predict mica Rb-Sr ages for slowly cooled rocks containing biotite, muscovite, and feldspar. Mica ages depend on the rock mode and usually differ from ages predicted by Dodson's model. Cooling rates derived from model ages by using the empirical difference in closure temperature between muscovite and biotite of 200 °C are similar to the cooling-rate input to the model for feldspar-rich rocks, but are higher for feldspar-poor rocks. In biotite-rich rocks, biotite ages older than muscovite ages are predicted. Mica Rb-Sr ages may give erroneous cooling paths, and variations in cooling path between adjacent areas could be an effect of sampling rocks with different modes.