Two new calcium thallates, CaTl2O4 and Ca2Tl2O5, have been isolated that crystallize in the Cmcm space group with a = 3.3255(1) Å, b = 11.022(1) Å, and c = 10.479(1) Å for the first and a = 3.3431(1) Å, b = 11.159(1) Å, and c = 13.499(1) Å for the second. The ab initio determination of their structure from powder X-ray data shows that they are closely related and that they represent the first two members of a series with the generic formula CanTl2On+3. In fact, the first member, CaTl2O4, is isotypic to CaTi2O4 and can be described as a chemical twin of [Tl2O4]∞ rock salt layers, whereas the second member, Ca2Tl2O5, is derived from the first by increasing the thickness of the rock salt layers, i.e., by replacing double ribbons of edge-sharing octahedra by triple ribbons. In both structures, the chemical twinning is induced by the ability of calcium to adopt the trigonal prismatic coordination in prismatic tunnels; the direction of rock salt layer is invariable, i.e., parallel to the (113) plane of the cubic rock salt structure.