1. The kinetics of isothermal decomposition of high-temperature β-phase for two-phase martensitic titanium alloys after undercooling from different temperatures are described by typical C-shaped curves; this decomposition may have one or two stages above the MS point. The type of isothermal decomposition diagram for this phase is governed by the alloy heating temperature and the temperature for the start of the martensite β→α″-transformation. 2. An increase in alloy heating temperature into the two-phase region leads to a shift in C-shaped curves in the direction of higher temperatures and shorter exposures, i.e., the decomposition process is accelerated. 3. With isothermal exposure for undercooled alloys in the martensite range it is possible for the martensitic β→α″-transformation to occur. With undercooling to a temperature above MS high-temperature β-solid solution is stabilized and the martensitic β→α″-transformation does not occur during subsequent cooling. 4. Mechanical properties of the alloys are goverved by the mechanism of high-temperature β-phase decomposition. Decomposition by an intermediate mechanism provides high alloy strength, although the ductility is lower. As a result of β-solid solution decomposition by a diffusion mechanism, an alloy has higher ductility with quite high strength. The optimum combination of strength and ductility as well as high property stability is obtained as a result of isothermal exposure for an alloy undercooled from the two-phase region to a temperature corresponding to the maximum rate for β-solid solution decomposition in the first stage.