In-depth experimental investigations did not allow for the preparation of binary RbTl. Mixed alkali metal approaches K1-xRbxTl suggest that the KTl structure type is favored up to x=0.69 while the CsTl type could not be realized in ternary approaches. In addition, in these cases, the formation of lower symmetry, monoclinic compounds underlines this experimental finding. The DFT calculated formation energies were used to investigate the trend between the two phases with increasing rubidium concentration. At the same time, the phonon calculations were used as an indicator of dynamical stability in hypothetical RbTl. The electronic structure calculation of rubidium in the KTl structure type suggests that an increasing rubidium content does not affect the electronic structure around the Fermi level. The preference for rubidium on the A1 crystallographic site is observed in experiment and also independently obtained in the calculated ground state energies. The discrepancy of the in theory dynamically stable but experimentally not observable binary RbTl might be explained by an effect of temperature. Future experiments will show, if the application of more sophisticated cooling and quenching techniques could facilitate higher concentrations of rubidium in A1-xRbxTl (A=K, Cs) compounds.