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 4f² electronic configuration. A new and concise measurement by a single parameter, σₑₑ, is introduced to quantify the overall strength of interelectronic repulsion, as the alternative to the Slater parameters, Fᵏ (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⁺ must 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 σee is only up to 5%, with the magnitude of σₑₑ in the order F⁻ > Cl⁻ > O²⁻ ≈ 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 σₑₑ (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. Copyright © 2013 American Chemical Society.