An in situ X-ray diffraction study of the precipitation reaction of a supersaturated solid solution single crystal of Ni-12.5 at.% A1 alloy was performed for moderately high temperatures. The diffusion-control coarsening model of Lifshitz and Slyozov is used to interpret the results of measuring the rate constants determined from the growth of the average particle size and volume fraction of the γ′ phase precipitate as derived from the line width and integrated intensity of the superlattice reflection, respectively. Both the diffusion coefficient and the interfacial energy between the matrix and precipitates were experientally determined for temperatures in the range of 550-700°C. The interfacial energies are reasonable when compared with literature values and they seem to indicate a decrease with increasing temperature. The diffusion coefficients yield consistent yet lower values when compared to results for Ni-self diffusion in single crystals, and to polycrystalline measurements of Al diffusion in Ni at comparable temperature. This difference is suggested to be related to the precipitate morphology change from sphere to cuboid evidenced in many Ni-base alloys.