The well-known nephelauxetic series of ligands describes the change in interelectronic repulsion of the central metal ion, which is reduced on going from the vapor to crystalline state. This study examines the trends and quantifies the mechanism of this series for the lanthanide ion Pr3+, with the 4f(2) electronic configuration. A new and concise measurement by a single parameter, sigma(ee), is introduced to quantify the overall strength of interelectronic repulsion, as the alternative to the Slater parameters, F-k (k = 2, 4, 6). Energy parameters have been derived from the literature electronic spectra of Pr3+, in the free ion and in various crystalline hosts, with new calculations in some cases. It is found that at least the first 12 of the 13 multiplet terms of Pr3+ be well-determined to obtain reliable parameter values. The shifts of various energy levels for changes in the Slater parameters are not uniform in direction. For the various Pr3+ solid-state systems, the change in sigma(ee) is only up to similar to 5%, with the magnitude of sigma(ee) in the order F- > Cl- > O2- approximate to Br- > C, and decreasing with lower coordination number of the ligand. The decreases of the Slater parameters from the free ion values are reasonably estimated by considering the dielectric constant of the medium. In particular, the magnitude of sigma(ee) (and of the spin-orbit coupling constant) is proportional to the polarizability of the ligand for F-, Cl-, O2-, and Br-. The data point scatter for oxide systems is accounted for by considering the individual ligand bond distances. A fair estimation of nephelauxetic effects can be made from some luminescence transition energies, by contrast with Eu3+ systems where crystal field effects also play a major role. In conclusion, the nephelauxetic effect of Pr3+ is due to the polarization of the ligand by one 4f electron, and the interaction of the other electron with the induced multipolar moments, of which the dipole moment dominates.