Summary: The effects of hydrothermal treatments on gel mixtures and synthetic minerals in the C₃S₂-M₃S₂ and C₂S-M₂S regions of the CaO-MgO-SiO₂ system have been investigated. The products formed in the temperature range 180-350°C at pressures up to 220 N/mm² were identified. The reactants generally behaved as simple mixtures of binary silicates and their products agreed well with existing data for reactions in the CaO-SiO₂-H₂O and MgO-SiO₂-H₂O systems. An unidentified phase that resulted from the hydration of åkermanite could be a new compound in the quaternary CaO-MgO-SiO₂-H₂O system. There was no evidence to indicate that a magnesium analogue of kilchoanite can be formed from materials that are isostructural with γ-Ca₂SiO₄.