The diffuse X-ray scattering from single crystals of Ni - 11.3 at.% Si and 12.3 at.% Si, both aged at 1293 K followed by a water quench, was measured at room temperature. The measured diffuse intensities were analyzed to determine the Warren-Cowley atomic short-range order (SRO) parameters α. The 17 values of α obtained were fitted in a 5 × 10 atom model crystal to simulate the SRO. It was found that the C16 and the C17 configurations are significantly enhanced in comparison to a random crystal. The pair-interaction energies V were obtained using an inverse Monte Carlo method from the α parameters. Assuming that V are independent of temperature, the α were calculated as a function of temperature on the basis of the Monte Carlo simulation. The curves of α for 11.3 at.% Si showed a knee point at 1262 (19) K and for 12.3 at.% Si at 1325 (11) K. Both temperatures are higher by about 100 K than those of the (fcc/fcc + L1) phase boundary in an equilibrium phase diagram. © 2007 Acta Materialia Inc.