A numerical index is proposed that ranks solvents according to their polarity. It is based entirely on structure, encoding the relative content of exterjacent electrons in the molecule. The index is the first-order valence molecular connectivity index, 1 sub chi upsilon. The index is modified for the number of isolated functional groups in the molecule. A comparison with solvent polarity indexes based on several experimental methods reveals a good relationship. The polarity index proposed can be quickly calculated, it does not depend on the availability of the actual molecule, and it permits prediction of solvent polarity or the polarity of mixtures. (Copyright (c) 1981 by the American Pharmaceutical Association.)