The mechanism of formation of (Ba1-xCax)TiO3 Mixed crystals using solid solutions of BaCO3/CaCO3 was investigated in detail by X-ray diffraction and thermochemical methods. As a result it has been shown that the mechanism of (Ba,Ca)TiO3 formation differs according to the nature of starting materials : BaCO3/CaCO3 solid solutions or a "classical" mixture of both carbonates.