The order-disorder phase transition LT- --> HT-Ni1.50Sn, which takes place in equilibrium at 782 K, has been studied in detail by annealing quenched HT-Ni1.50Sn at T less than or equal to 673 K. Two main stages of the transformation towards the LT-phase, which occur on different time-scales, have been identified. First, the axial ratio. c/a of the hexagonal HT-Ni1.50Sn increases, and only during a later stage, the full superstructure with a characteristic orthorhombic distortion develops. The activation energy for the first stage has been determined to be 165 kJ/mol by monitoring the time-dependence of the axial ratio c/a by ex- and in-situ powder diffraction.