Abstract Results of our research programme on the vibronic spectroscopy of rare earth ions are summarized. They relate mainly to Pr 3+ and Gd 3+ . In both cases the vibronic intensity increases with increasing degree of covalency. The higher vibronic transition rates for Pr 3+ are ascribed to the larger extension of its 4f orbitals. The Δ process gives an important contribution to the vibronic intensity, especially for Pr 3+ . Further, the vibronic intensities of isoelectronic Gd 3+ and Eu 2+ are compared. The influence of the second coordination sphere is larger than expected at first sight. Concentration enhancement of vibronic intensities and line narrowing in two-photon spectra are fake effects due to saturation.