The influence of alloying elements and heat treatment on the structure, phase composition and hardness in two series of cast alloys of the Ti–Nb–Si system with a silicon content of 1 and 1.2% wt. and different niobium content from 10 to 18% wt. is studied. As shown, in as-cast alloys of this composition the niobium content in some areas varies in a fairly wide range because of the liquation, respectively, a heterogeneous and nonequilibrium structure is formed, which consists of metastable phases: $\alpha^{'}$, $\alpha^{''}$, $\beta$, dispersed (Ti, Nb)$_{5}$Si$_{3}$ and (Ti, Nb)$_{3}$Si silicides. The maximum hardness of the experimental as-cast alloys coincides with the highest content of $\alpha^{'}$ or $\alpha^{''}$ phases and is due to both the doping of the solid solution of these phases and the presence of dispersed silicides in them. As found, increasing the niobium content to 18% wt. in as-cast alloys Ti–$x$Nb–(1–1.2)Si leads to a decrease in the solubility of silicon in the $\alpha^{''}$ phase and additional precipitation of silicides, as well as an increase in the amount of $\beta$ phase, and, consequently, reduced hardness. Heating and holding of as-cast alloys on heat treatment brings them to a more equilibrium state, which is preserved at low cooling rates. Dispersed metastable (Ti, Nb)$_{5}$Si$_{3}$ silicides dissolve, the existing stable (Ti, Nb)$_{3}$Si silicides grow, and new ones are formed, thus increasing their number. That reduces the hardness compared to the as-cast state. It is found that dispersed, newly formed by cooling silicides do not contain niobium and the last one dissolves in silicides during their growth, so large silicides have a composition of (Ti, Nb)$_{3}$Si. As shown, the transition from $\alpha^{'}$ to $\alpha^{''}$ structure in the experimental alloys depends not only on alloying but also on the cooling rate. As the cooling rate increases, the $\alpha^{''}$ phase of different degree of doping is formed, and its orthorhombicity is 0.99–0.96 depending on the niobium content.