The phase separation in a supersaturated Ni-12.5 at.% A1 alloy has been investigated by highangle diffuse scattering, small-angle scattering, transmission electron microscopy and electrical resistivity methods. High-angle X-ray diffuse scattering measured at room temperature on a bulk single crystal homogenized and quenched from 1323 K showed well developed short-range atomic ordering. An electrical resistivity isochrone shows a marked peak at about 675K. Small-angle scattering measurements on samples annealed at 823 K clearly indicate that, at this temperature, phase separation occurs by a nucleation and growth process. It has been argued that a cluster coagulation model well represents the processes occurring at 823 K. It is, however, not required that the same process be operational at lower temperatures, particularly below the break in the resistivity isochrone. Measurements for annealing times up to 840 h at 575 K reveal weak small-angle scattering which has some of the characteristics of a continuous transformation. However, much longer transformation times at the lower temperature will need to be investigated before the possibility that this is only a kinetic effect associated with slow nucleation of the second phase can be ruled out.