For the purpose of obtaining more detailed information on the phase transformation of metastable beta-type titanium alloys, the phase changes of Ti-Fe, Ti-Cr, Ti-Mn and Ti-Mo binary alloys during isothermal transformation were investigated by means of the microscopic observation and X-ray diffraction. The results obtained are summarized as follows: (1) Before the precipitation of the alpha phase, an unusual microstructure was observed in the specimens which were held for a short time at a given higher temperature prior to quenching. This unusual microstructure seemed to be caused by subgrain boundaries, etch pits and titanium hydride. (2) In the TTT diagrams obtained it was found that the starts of the beta-to-alpha and the beta-to-omega reactions shifted towards longer time and lower temperature as the solute concentration increased. (3) In each alloy system, the beta-to-alpha and the beta-to-omega transformations were observed. However, the beta-to-low concentration beta transformation below about 300°∼250°C, reported by Harmon et al.⁽¹⁷⁾, was not observed. (4) In the specimens isothermally transformed above about 600°C the alpha phase precipitated predominantly at the grain boundaries, while in the specimens transformed below about 550°C the alpha phase precipitated in the grains. (5) Looking through the TTT diagrams of the alloys which have an equal atomic concentration, it was found that the rate of precipitation decreased in the order of Ti-Cr, Ti-Fe, Ti-Mn and Ti-Mo alloys. This did not coincide with the order of diffusion rate in the beta-titanium⁽¹⁸⁾. In the case of eutectoid type alloy, referring to the phase diagrams of such binary alloys, it was concluded that the rate of precipitation became faster when an eutectiod temperature was getting higher or an eutectiod composition was getting lower.