The Onsager reciprocal relations are essential to multicomponent transport theory. A discussion of the principles that should be used to derive flux laws for coupled diffusion is presented here. Fluctuation theory is employed to determine the reciprocal relation for transport coefficients that characterize coupled mass and heat transfer in binary ideal-gas mixtures. This example illustrates the statistical basis of reciprocal relations among macroscopic transport properties. (Contains 1 figure and 4 footnotes.)