Experimental and theoretical considerations indicate that the distribution coefficient for iron and magnesium between coexisting garnet and cordierite increases with temperature in the assemblage cordierite-garnet-sillimanite-quartz. This conclusion is confirmed by distribution coefficients from natural garnet and cordierite from geologically well defined settings. The only published calibration which incorporates this feature is that of Currie (1971), and this is the only calibration which can be qualitatively correct although it may be wrong in detail. Other calibrations encounter catastrophes, particularly in andalusite-bearing assemblages.