We report a Li-7 and P-31 nuclear magnetic resonance (NMR) study in the Li1-3xMgFexPO4 phases between 150 and 410 K. This study, complementary to those made using Mossbauer and magnetic neutron diffraction experiments, confirms that the Fe ions enter as Fe(III) in the lattice, and that they enter substituting Li ions. Ionic conductivity measurements, together with the NMR behavior of the Li-7 and P-31 NMR spectra show that no Li mobility occurs in the temperature range studied even with the addition of the Fe impurity. (C) 1998 American Institute of Physics.841416421