Phase instabilities of binary Ni2Cr and Ni3Cr alloys after long-term exposure at temperatures between 450 and 600 degrees C were studied by monitoring lattice parameter, electrical resistivity and microhardness variations and by analysing the microstructural evolution of the alloys at interrupted annealing times. Both materials undergo two metallurgical changes : atomic ordering, based on the Ni2Cr superlattice, and the precipitation of the alpha’-Cr phase. Short-range order develops in both alloys during the first’s hours of ageing. The degree of order and kinetics of ordering transformation depend on the alloy composition, time and temperature of ageing. In Ni2Cr, the short-range ordered structure transforms to long-range order after ageing for 100-1000 h below 525 degrees C, but in Ni3Cr the transformation occurs after 30 000 h. The embrittling alpha’-Cr phase precipitates at grain boundaries in both alloys after long-term ageing, mainly at 550-600 degrees C. The amount and size of particles increase with time and temperature of ageing.