The reverse martensite transformation in two stainless steel alloys has been investigated; one is a pure ternary alloy (Fe -16% Cr -12% Ni) and the other also contains molybdenum and carbon (Fe -15% Cr - 8½% Ni 2% Mo 0.1% C). Both alloys are fully austenitic at room temperature after solution-treatment but undergo partial transformation to martensite when quenched to -196°C or deformed at room temperature. The transformation has been studied by room temperature measurements of specific saturation magnetic intensity, by hardness measurements and by transmission electron microscopy after heating the partially martensitic specimens for short times (1 or 2 minutes) in molten salt baths. Reversion during slower rates of heating has also been studied by a differential dilatometry technique and by elevated temperature measurements of dynamic Young's modulus. The results suggest that the reverse martensite transformation in these alloys occurs mainly by a diffusionless (shear) mechanism but that diffusion-controlled reversion and, possibly, stabilisation effects are also present when slower heating rates are employed. The 'reversed austenite' contains a high density of tangled dislocations together with stacking faults and reversal twins and suggestions have been made regarding the formation of these structural imperfections in the austenite. Strengthening and stabilisation effects produced by repeated transformation cycling, γ⟶∝'⟶γ , have also been studied. The process of 'annealing-out' of the reversed austenite structure in the ternary alloy has been studied and it is suggested that this occurs by a continuous recovery mechanism rather than by the movement of high angle boundaries through the deformed austenite.