Titanium alloys have long been recognized as outstanding strong, light and corrosion resistant alloys. TC21 alloy has recently received considerable attention due to its high strength, toughness, damage-tolerance properties and low crack propagation rate. Similar to steels, titanium alloys have demonstrated their capacity to produce martensitic microstructure upon suitable heat treatment. Their martensite and its transformations upon subsequent heat treatment proved to be an important tool to obtain controllable properties. The martensite (start) characteristic temperature (Ms) has received some attention as regards its dependence on composition. On the other hand, no similar attention was given to the dependence of the other important martensitic (finish) characteristic temperature (Mf) on composition. In view of the foregoing, this work was thus planned to fulfill this lacking information via subzero hardening treatments of TC21 α/β alloy. Detailed analysis of the so obtained microstructures via optical and scanning microscopy, and x-ray diffraction data has led to a quantitative estimation of the Mf temperature and its composition dependence. Additionally, the hardening effect of those subzero hardening treatments was studied via hardness and microhardness measurements. Significant findings were recorded which are expected to help reaching useful property levels such as strength, wear resistance and damage-tolerance.