The solid solution series 4CaO·3Al2O3·3H2O – 4CaO·3Ga2O3·3H2O was synthesised by means of hydrothermal treatment. The products were characterised by X-ray powder diffraction and Fourier-transform infrared spectroscopy. No miscibility gaps were observed. All products are orthorhombic, having space group Aema. The main structural change due to the substitution of Ga for Al is an increase in the average T–O bond length and the lattice parameters. The coordination of Ca by oxygen is only slightly affected. The increase in the T–O bond lengths is compensated for by a decrease of the inter-tetrahedra angles, leading to an adaptation of the layer-geometry to the coordinational needs of Ca.