The phase separation at 550 °C in a Ni-12.7 at. pct Al alloy was investigated by means of small angle X-ray scattering, large angle X-ray scattering, electrical resistivity, and transmission electron microscopy. It was found that an abrupt change in the electrical resistivity and small angle scattering occurred within 15 seconds at 550 °C, following quenching from 1060 °C, and transmission electron microscopy confirmed the presence of small nuclei. However, after this period of rapid change, there was an incubation period of about ten hours in which there was minimal net growth of the nuclei. After the incubation period, growth was accelerated. High angle side bands were observed after a few minutes annealing, and their development closely paralleled the growth of the nuclei and precipitate particles at this temperature. For times beyond the incubation period, small angle X-ray scattering diagrams from single crystals became markedly anisotropic, indicating a quasi-regular spatial distribution of particles. The large and rapid change, in both electrical resistivity and in X-ray small angle scattering, observed after annealing at 550 °C of samples quenched from 1060 °C was attributed to the formation of tiny regions of short range ordered Ni3Al which constitute subcritical nuclei.