We present novel results on the mechanism of the α-β structural phase transition (STP) occurring in FePO₄. High accuracy x-ray diffraction experiments followed by a structural analysis provide us with precise information on the thermal disorder change on different atomic sites. The data are analysed in the light of lattice dynamics simulation results. The rigid unit mode (RUM) approach of the dynamics simulation allows an understanding of the anisotropic displacement parameters (ADPs). Surprisingly, the role of critical fluctuations of the order parameter, in the sense of Landau and Lifshitz theory, appears not to be relevant in the case of this SPT and the understanding of the dynamics requires a knowledge and analysis of the microscopic details of the system.