The decomposition of a Ni-12.5 at.% Si alloy single crystal was investigated by means of a high temperature in situ X-ray diffraction study. This study entailed the use of a Gorskii-Bragg-Williams thermodynamic model which provides a means of predicting possible decomposition mechanisms within the two-phase equilibrium region of a binary alloy having an fcc disordered matrix plus ordered domains with an Ll (CuAu type) superstructure. In order to determine if a unique ordering "instability" temperature can be associated with the development of the γ′ (NiSi) phase, various heating rates were used while monitoring the growth of the 100 superlattice reflection of an initially disordered specimen. This study showed that the onset of the γ′ phase growth depends upon the heating rate and no unique "instability" temperature exists, therby suggesting that nucleation and growth processes govern the decomposition behavior. This result was supported by cyclic heating studies in which the growth of the γ′ phase was observed to depend upon thermal history as predicted by the diffusion-controlled growth mechanism. Furthermore, the particle size and volume fraction growth rates of the γ′ phase were found to obey the t 1 3 and the t -1 3 law, respectively. These observations are again consistent with the diffusion-controlled coarsening kinetics. © 1986.