Parameters which claim to reflect the Lewis-type donor properties of s
olvents are reviewed. It will be shown that the original concept of Le
wis, where one partner supplies one electron-pair for a chemical bond
and the other partner accepts this electron-pair, is limited to ''hard
-hard'' interactions. ''Soft-soft'' interactions, where both solute an
d solvent donate and accept electron pairs cannot be accounted for by
parameters derived from model substances, which are based on the class
ical Lewis concept. Disregard of the different chemical behaviour of '
'hard-hard'' and ''soft-soft'' interactions has led to misinterpretati
on of solvent parameters and to deviations in correlations of kinetic
and thermodynamic properties with solvent parameters.