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Martensitic transformations in equiatomic titanium-nickel and titanium-nickel based alloys are of interest because they are associated with the shape memory effect. Equiatomic titanium-nickel, which possesses such superior mechanical properties as high strength, elevated ductility and corrosion resistance, is the most important of several shape memory alloys. Here, the character of the martensitic transformation in equiatomic titanium-nickel alloy with some of the nickel replaced by ruthenium is discussed. Data are presented on the temperature of the transformation upon heating and cooling, the transformation sequence and the stabilisation of the high temperature phase in TiNiTiRu alloys, using electrical resistance, thermal expansivity and differential thermal analysis between ∼ 300°C and liquid nitrogen temperatures. Alloys which contained 0.5 to 2 atomic per cent ruthenium were found to undergo a two stage transformation, while the high temperature phase in titanium-nickel-ruthenium, which has B2-type crystal structure, was stabilised by the addition of 2 atomic per cent ruthenium.