Yb3+ doped LaSi3N5 phosphors have been prepared by solid-state synthesis in a high temperature furnace directly coupled to a glove box. The charge transfer (CT) band of Yb3+ in LaSi3N5 was observed at an unusually low energy of 3.1 eV. Excitation of this CT band results in f–f emission of Yb3+ at 1.2 eV, which could make Yb3+ doped LaSi3N5 suitable for application as a spectral conversion material for infrared light-emitting diodes or solar cells. Substitution of La3+ by the smaller Y3+ ion shifts the CT to higher energy (3.9 eV). A clear relation between the CT energy and the Yb3+ f–f emission intensity was found. With the help of a detailed energy level diagram that contains the position of the 4f and 5d levels of all divalent and trivalent ions with respect to the valence and conduction band, all hitherto observed luminescence properties of divalent and trivalent lanthanide dopants in LaSi3N5 are reviewed and explained.