In the experimental determinations of the coefficients of diffusion of impurities in semiconductors reported to date, it has usually been assumed that these coefficients do not vary with concentration. This assumption is questioned here. Interactions between acceptors, donors, electrons, and holes may lead to complicated diffusion equations, as shown by an analysis based on Onsager's theory. In particular, appreciable covalent compound formation is likely to occur between some substitutional donors and acceptors. This alone may lead to a marked dependence of diffusion coefficients on concentration, and to diffusion of acceptors induced by concentration gradients of donors and vice versa. Such effects are suggested by some discrepancies in the experimental results reported thus far.