Two calcium aluminate carbonate hydrates, previously reported in the literature, were prepared and various properties determined. The compound 3CaO·Al2O3·CaCO3·11H2O, was prepared by precipitation from mixtures of solutions of calcium aluminate, calcium hydroxide and sodium carbonate; and, in less pure form, by several other means. It forms thin hexagonal plates having refractive indices 1.532 and 1.554. From X-ray diffraction, cell parameters, calculated on the assumption of hexagonal symmetry, are a=8.716, c=7.565. On continued exposure to atmospheric carbon dioxide it is decomposed to calcium carbonate and hydrated alumina. The compound 3CaO·Al2O3·3CaCO3·32H2O, was prepared by precipitation from mixtures of ammonium bicarbonate, calcium aluminate, and calcium hydroxide in aqueous sucrose solution. It crystallized as spherulites of minute needles. Other methods produced individual crystals, some tubular, but only in minute quantities or mixed with other phases. X-ray diffraction indicates isomorphism with 3CaO·Al2O3·3CaSO4·32H2O. 3CaO·Al2O3·3CaCO3·32H2O is, in general, less stable than the monocarbonate complex. The needle-form "silicoaluminate" reported by Flint and Wells was re-examined. It was concluded that the phase probably contains CO2 as well as SiO2, but the exact composition is in doubt.