The kinetic behavior of precipitation in a supersaturated Ni-12.5 at.% Si alloy single crystal has been studied by an in situ high-temperature X-ray diffraction method for the first time. Experimental and analytic techniques have been developed to simultaneously determine the interfacial free energy, γ, the solute diffusivity, D, and the equilibrium solubility, C, from a single type of X-ray measurement The principle of analysis entailed a diffusion-controlled coarsening model put forth by Lifshitz and Slyozov, and independently by Wagner. The kinetic parameters, i.e. rate constants, governing the growth of the particle size and the volume fraction of the γ' precipitate are derived from the line width and the integrated intensity of a superlattice reflection of the γ' phase, respectively. Results for D in the temperature range from 400° to 550°C are in good agreement with the extrapolated values obtained from the high-temperature data. Enhanced diffusion is noticed at temperatures below 500°C. Reasonable values for the interfacial free energy (about 11 ergs/cm) are found for all temperatures considered. The equilibrium solubility is found to be consistent with the published phase diagram values. © 1985.