Quantitative measures of the capability of solvents for interaction with solutes. Such parameters have been based on numerous different physicochemical quantities, e.g. rate constants, solvatochromic shifts in ultraviolet/visible spectra, solvent-induced shifts in infrared frequencies, etc. Some solvent parameters are purely empirical in nature, i.e. they are based directly on some experimental measurement. It may be possible to interpret such a parameter as measuring some particular aspect of solvent–solute interaction or it may be regarded simply as a measure of solvent polarity. Other solvent parameters are based on analysing experimental results. Such a parameter is considered to quantify some particular aspect of solvent capability for interaction with solutes.