Low-temperature electronic absorption and emission data are reported for Tm3+ at the octahedral site in crystals of Cs2NaTmCl6. Thirty-seven crystal field levels (total degeneracy 88) of the f(12) configuration (total degeneracy 91) have been assigned, and in several cases the levels are split due to electron-phonon coupling interactions. The fitting of the energy levels, using the conventional f(12) analysis with 12 variable parameters, gives a mean deviation of 53.3 cm(-1). This is reduced to 9.3 cm(-1) by including the 4f(12)np(6)/4f(13)np(5) configuration interaction, using 16 variable parameters. The results indicate a tendency for the early members in the series of Ln(3+) ions to interact with the p-electron, and the later members with the p-hole, configurations, following the redox properties of the ions. The interacting configuration is charge-transfer (n = 3) rather than metal ion (n = 5), and the mixing of ligand wave functions with those of the metal ion may be responsible for the unusually strong electron-phonon coupling identified for several electronic states of Tm3+ in TmCl63-.