Temperature-modulated transmission spectra in the region of the 4d→4f excitations in LaF3 show a variety of effects, none of which can be explained by the free-atom model hitherto used to describe this absorption. The sharp lines below threshold are shown to broaden with increasing temperature and to shift slightly. The broad ‘‘autoionization’’ region is shown to be composed of several structures, only one of which is included in the atomic picture. The effects of the lattice on these transitions ‘‘within’’ the La3+ ion are significant.