—A melt inclusion study was carried out in the leucite-bearing tephriphonolite and phonolite lavas of the Bolsena complex in order to obtain direct data on the chemical composition of initial melts, their evolution, and their crystallization temperatures. It has been found that the initial melt for the considered rocks was of tephrite–basanite composition. Its crystallization began with the formation of clinopyroxene phenocrysts at 1205–1100 °C, then leucite and plagioclase crystallization took place at about 1120 °C and 1080–1060 °C, respectively. The initial tephrite–basanite melt was slightly enriched in volatile components (H2O, F, SO3, and Cl). During the crystallization of clinopyroxene, leucite, and plagioclase, the composition of the initial magma changed toward an increase in the contents of SiO2, Al2O3, and K2O and a decrease in the contents of FeO, MgO, and CaO, i.e., evolved toward phonolite. A similar evolution trend is typical of alkaline basic systems. The tephrite–basanite melt was probably the product of the crystallization differentiation of the parental mantle magma similar in composition to the leucite-bearing tephrite–basanite of the Montefiascone complex.