The activities in liquid Ni–Ti alloys were experimentally measured by the Langmuir vaporization method. By using measured rates of evaporation and temperatures, the activities of nickel and titanium were calculated by applying Hertz–Knudsen equation and a regular solution model. An electron beam vacuum furnace with a water cooled copper crucible was used for evaporation experiments to avoid contamination by oxygen and gaseous elements. Temperature was measured by two color optical pyrometer. The rates of evaporation in pure substances were measured to verify Hertz–Knudsen equation with the evaporation coefficient α of unity. Rates of evaporation in Ni–Ti alloys were then measured. This system was assumed to be a regular solution. Consequently activities of nickel and titanium were calculated to be about 0.04 at 50 at% at 1773 K. The activities obtained in this study were ranged between those in two studies in literatures. Results of this study agree reasonably with those for Ni–Al alloys in literatures. This system exhibits strong negative deviations from ideality. Moreover it was shown the possibility of thermodynamic measurements for the other systems which contain high reactive elements.