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The changes in the short-range ordered structure of an alloy containing 74 at. % Ni, 26 at. % Fe have been determined for a series of anneals between 0 and 40 h at 480°C. Three-dimensional short-range-order parameters have been determined by diffuse X-ray scattering analysis from a single-crystal sample. The ordered arrangement has been shown to change from one dominated by nearest-neighbor interactions to one with longer range interactions in the period between 8 and 12 h at 480°C. For longer times of annealing, this gradually changes to a more cooperative interaction extending over longer distances and approaching the arrangement found in the fully ordered structure. The changes in the hardness, resistivity, and coercive force which occur at or near 12 h of annealing support the contention that a change in the type of atomic arrangement is taking place at this time.