The crystal structures of thallium titanyl phosphate TlTiOPO4, have been determined at room temperature, at 500 degrees C, and at 650 degrees C, by using time-of-flight neutron single crystal diffraction. The structures below and above the ferroelectric to paraelectric phase transition (T-c = 583 degrees C) are compared : the data suggest a displacive second order phase transition which is dominated by thallium ion motion, but is also accompanied by subtle displacements of the framework atoms, during which, one of the two distinct TiO6 octahedra regularizes to (1) over bar symmetry.