The effect of chemical composition of Ti-0.4 Al α-titanium alloy (PT-1M alloy) on the phase transformation temperatures up to melting temperature has been investigated using ThermoCalc software. The possibility of appearance of Ti2N nitride, Tix(CN)-type carbonitride, and Ti2Fe-type intermetallic in the alloy structure in equilibrium state has been proved by thermodynamic calculations. It has been proposed the so-called aluminum equivalent (Aleq) for PT-1M alloy to analyze the effect of chemical composition on the phase transformation temperatures change. The following equivalents have been suggested: Al eq Tβ for β ↔ ( β + α ) − transformation, Al eq T l i q for L ↔ β − transformation, and Al eq Tsol for ( L + β ) ↔ β − transformation. It has been shown the linear correlation between the characteristic temperatures of phase transformations in the investigated alloy and the proposed aluminum equivalents.