Energy-resolved optically stimulated luminescence (OSL) spectra and thermoluminescence (TL) glow curves of a powder sample of YPO 4:Ce 3+,Sm 3+ were measured to investigate the nature of the trapping centre and to locate its energy level relative to the valence and conduction bands of the YPO 4 host. The high-temperature glow peak could unequivocally be assigned to Sm 2+ (thus Sm 3+ acts as an electron trap). The trap depth of this centre, as derived from the OSL excitation spectra, is in good agreement with the Dorenbos model prediction. The OSL excitation spectra also reveal excited states of Sm 2+ well below the conduction band. These excited states produce a broadening of the high-temperature TL glow peak and also cause the activation energy determined by the Hoogenstraten method to underestimate the trap depth.