Abstract Co Ni solid solutions, such as Co10Ni20, Co40Ni40, Co50Ni50, Co60Ni40 and Co70Ni30 alloys, have been produced by mechanical-alloying using Co and Ni powders prepared in our laboratory by the polyol process as metallic precursors. The grinding transforms the hexagonal Co phase into a cubic Co phase by stacking fault creation and permits a possible mutual diffusion of Co and Ni species in the two f.c.c. phases. The resulting powder alloys are nanocrystalline with a high atomic-level of internal strain. Magnetic data on these powders shows behaviour typical of a soft ferromagnetic alloy. A strong influence of oxygen contamination on magnetic properties of these alloys was observed, mainly on the saturation magnetization which decreases with increasing oxygen content.