Ordering and phase separation in Ni-Cr-Al alloys were studied using Monte Carlo (MC) simulation and three-dimensional atom probe. Excellent agreement was found. Simulation showed that. (i) in the gamma' phase, Cr substitutes for both Al and Ni sublattices; (ii) in the gamma phase, a "Ni3Cr"-type and an Ll(2)-type short range order (SRO) develop, and transient Al-rich Ll(2) ordered zones exist. In low supersaturated alloy a nucleation and growth mechanism was observed. Simulations indicated that phase transformation occurred in three stages: I-SRO develops: a "Ni3Cr"-type and an Ll(2)-type; II-formation of Al enriched and ordered zones; III-growth and coarsening of gamma' particles. Simulations of high supersaturated alloys showed that a high density of Ll(2) ordered zones nucleates before unmixing. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights received.