Alloys ranging from 18 to 60 wt% molybdenum were prepared and homogenized in a hydrogen atmosphere at 1250 deg C for a minimum of 50 hr. Subsequent heat treatments in the temperature range 700 to 1000 deg C provided information on the progress of phase transformations in the system and, in so far as possible, indicated the equilibrium phase relationships to be expected. The lattice Parameter of the alpha solid solution as a function of molybdenum concentration was determined and used as an aid in establishing the extent of solid solubility of molybdenum in nickel. Molybdenum expands the nickel lattice at the approximately constant rate of 0.0033 A per weight per cent molybdenum. The peritectoid reaction for the formation of the BETA phase from the alpha and gamma phases was established at 868 plus or minus 2 deg C, which compares to a previously reported temperature of 840 deg C. The body-centered tetragoral structure of the BETA phase was confirmed, although there is evidence for an intermediate stracture to form during the process of transformation of the alpha to the BETA phase. The etching characteristics of the BETA phase were observed to be quite variable, depending on the etchant andmore » the rate at which the BETA has formed from alpha . In general, etching tends to produce a Widmanstatten pattern presumably associated with the large number of orientations available to BETA as it forms. Alloys in the composition range between the solubllity limit of molybdenum in nickel and the BETA phase composition develop Widmanstatten structures similar to the pure BETA phase. The peritectoid reaction for the gamma phase was observed at 910 deg C, which compares to the previously reported 890 deg C. The gamma phase does not possess a simple close- packed hexagonal unit cell as reported earlier. Diffraction studies showed a large number of faint lines which could not be analyzed. Observations indicate that the alpha phase transforms through one or more intermediate phases in approaching the equilibrium state for the alloy. The sequence frequently involves a precipitation from the alpha phase, followed by a grain-boundary- nucleated growth process yielding the equilibrium phases. (auth)« less