Titanium aluminides and alloys based on them are promising heat-resistant materials demanded in critical equipment and units, particularly, in shipbuilding, mechanical and aerospace engineering. The main method of obtaining the necessary complex of mechanical properties of intermetallic alloys is thermomechanical processing of them, the parameters of which can be obtained based on the construction of a phase diagram. The construction of experimental thermokinetic diagrams of intermetallic alloys is rather difficult. Based on the methodology of CALPHAD, the design technique for equilibrium state diagram and thermokinetic diagram of the transformation of intermetallic alloy of a Ti–Al system is proposed. The influence of aluminium on the temperature of titanium intermetallide Ti$_3$Al formation is determined. As shown, with an increase of aluminium content in the Ti$_3$Al intermetallide from 10 to 29 at.%, the onset temperature of the $\beta$-Ti $\to$ Ti$_3$Al transition increases from 520 to 1170°C. A further increasing of aluminium content in the intermetallic compound from 29 to 40 at.% leads to a slight decrease in the initial transition temperature to 1140°C. The developed technique can be used to simulate thermokinetic diagrams of anisothermal transformations in complex titanium alloys.