The different mixtures of cesium and rubidium lead mainly to the formation of Cs8-xRbxTl11 phases and alkali metal. Contrary to the potassium-rubidium mixture, no solid solution in the CsTl structure type was found, but instead the new compounds Cs1-xRbxTl (x=0.18, 0.42), which crystallize both in the monoclinic space group C2/c could be isolated from the mixture. The compound with 42% rubidium forms in samples with a rubidium proportion of 33-50%. In contrast the one with 18% rubidium only forms with a high excess of both, cesium and rubidium (see Cmce in the KTl structure type and contain compressed [Tl6] 6-octahedra as thallium substructure.