Abstract Under ultraviolet excitation cerium pentaphosphate CeP 5O 14 shows emission with a quantum efficiency almost independant of the Ce 3+ concentration and temperature. Energy transfer between the Ce 3+ ions is not efficient. Decay measurements have been performed and Ce 3+ decays with τ 1 = 13 10 −9s and τ 2 = 41 10 −9s independently of Ce 3+ concentration and temperature. The electron spin resonance spectra of Gd 3+ in Ce 1−xGd xP 5O 14 twin-free crystal are reported and the angular variations of the absorption lines have been fitted to the spin Hamiltonian corresponding to Cs symmetry. Twin-free crystals, selected by EPR measurements, exhibit only the τ 2 lifetime, τ 1 can be induced mechanically in the course of the crash the initially twin-free crystal.