An IR-spectroscopic study has been carried out on the solvation state of the model compoundEt3PO in the solvents methanol, ethanol, propanol, 2-propanol, butanol, 1,2-ethanediol, 1,3-propanediol, water, acetic acid, and formamide. With the exception of formamide (where only one solvate species was detected) the spectra show the presence of two basic types of solvate complexes, which derive from hydrogen bonded complexes withprimary coordination numbers of one and two, respectively. In water there is evidence that higher coordinated solvates (withprimary solvation numbers of two and three) are formed. Detailed informations about the structure of these complexes were obtained by spectrophotometric titrations, which show that extensive outer-sphere interactions are taking place in the neat solvents. For solvents, where only one kind of solvate species is present, an excellent correlation was found between solvent acceptor numbers and the wavenumbersv(PO) of the P=O stretching vibration ofEt3PO. By contrast, acceptor numbers of protic solvents usually represent average values, which are determined by the relative concentrations and31P-chemical shifts of the individual solvates present in solution. By means of the wavenumbersv(PO) of these solvates and the linear correlation mentioned above, it has become possible to calculate refined acceptor number values termed “Individual Acceptor Numbers”, which characterize the acceptor properties of protic solvents as a function of the solvation numbers of the substrates under consideration. It is shown, that the use of these individual acceptor numbers in linear energy relationships allows a significantly improved unified description of solvent effects on chemical reactions and physical processes in aproticand protic solvents.