The behavior of the melting curves of anhydrous silicates in the presence of water is examined in T-C-w and P-T diagrams. It is demonstrated that the effect of water on the melting temperatures of minerals can be explained with the use of the constants of the melting reactions under anhydrous conditions, with melt components assumed to be oxides recalculated to one cation. The water effect is thereby taken into account through a decrease in the fractions of melt components in compliance with the ideal solution model. The general form of the equation for the dependence of the melting temperature on the water concentration in melt satisfactorily reproduces the configurations of the liquidus curves. The example of the olivine-melt equilibrium is employed to demonstrate that the ideal model is able to quantitatively explain the extent of the melting temperature decrease when water is added. In order to adequately describe the experimental data, it is proposed to use a formal parameter: the effective water concentration, which takes into account the presence of the molecular and hydroxyl modes of water occurrence in the melt.