The effects of step-quenching on the microstructure and mechanical properties of a binary Ti-10Mo (wt%) alloy were investigated by transmission electron microscopy. The step-quenching treatment, consisting of solution treatment in the beta phase field at 850 A degrees C followed by step-quenching to the alpha/beta two-phase region at 650 A degrees C and holding for 0.5 h, was employed before water-quenching the alloy to room temperature. Direct quenching from 850 A degrees C to room temperature with or without an aging step at 650 A degrees C was also conducted for comparison. Microstructural observation revealed that step-quenching favored the formation of alpha precipitates thinner than 20 nm in the absence of alpha aEuro(3) or omega heterogeneous nucleation agents and effectively moderated the subsequent alpha aEuro(3) martensitic transformation by increasing the stability of the beta phase. Step-quenching generated a multiphase microstructure comprising alpha aEuro(3), beta, omega, and alpha phases by balancing the competitive martensitic alpha aEuro(3) and diffusional alpha transformations. Only alpha aEuro(3) martensite was formed in the beta matrix after direct water-quenching; the mixture of alpha aEuro(3) + beta phases was transformed to a lamellar alpha + beta microstructure with 5 min aging. The kinetics of alpha precipitation was calculated to illustrate the temperature dependence of alpha precipitation behavior during step-quenching. Direct water-quenching produced a tensile strength of 688 MPa and 36% ductility. After aging, the tensile strength was increased to 837-867 MPa, while the ductility was decreased to 5%. By step-quenching, the high tensile strength of 790 MPa and ductility of 23% were achieved.