The equilibrium phase diagram of the nickel-manganese system is determined between 500 and 850 °C, in the composition range between 25 and 70 at. % Ni. A combination of electron probe microanalysis, x-ray diffraction, and optical microscopy was employed to analyze 47 samples that were annealed anywhere from three to seven months. The equiatomic, antiferromagnetic, L10-NiMn phase that is of considerable technological interest was found to exist continuously between 500 and 700 °C. No other intermediate phases were found in this study at low temperatures. These results are in contrast to the currently accepted phase diagram published in most handbooks. A hot-isobaric-pressing method was used to initially bond samples that were subsequently used to determine interdiffusion coefficients in the Ni–Mn system at 650 °C. The Boltzmann–Matano method [T. Heumann, Z. Phys. Chem. 201, 168 (1952)] allowed the calculation of these interdiffusion coefficients across the α-Mn, β-Mn, γ-Mn, L10-NiMn, and γ-Ni phases. © 2002 American Institute of Physics.