Single crystals of RbBi2 and CsBi2 were grown using a Bi selfflux method. Pieces of bismuth (Alfa Aesar, 99.99%) and rubidium (Alfa Aesar, 99.9%) were put in an alumina crucible inside an Ar-filled glovebox. Cesium was first heated above its melting point by warming it in hand and then poured dropwise into the crucible and covered with Bi pieces. Crucibles were then put in fused silica tubes along with plugs of quartz wool, and the tubes were subsequently evacuated, back-filled with Ar, and sealed without exposing alkaline metals to air. They were then heated to 550°C, kept at that temperature for 8 hours, and then slowly cooled (5°C per hour) to 310°C at which temperature the excess Bi was spun off with the aid of a centrifuge. Clusters of octahedrally and tetrahedrally-shaped crystals were obtained, with sizes ranging up to ca. 1 mm. The phase identity of the crystals was examined using powder x-ray diffraction (PXRD) measurements employing a Bruker D8 FOCUS diffractometer with a Cu Kα radiation source and a LynxEye detector. A few single crystals were crushed for the PXRD measurements inside an Ar-filled glovebox and mixed with an Apiezon-N grease or petroleum jelly. Since the powdered samples quickly oxidize, reacting with oxygen and moisture from air, the scanning time had to be reduced to 8 min. PXRD patterns were then analyzed by means of the LeBail method using the FullProf software package . PXRD patterns taken after a prolonged exposition to air show that both compounds decomposed completely yielding weakly crystalline elemental Bi and an amorphous product (most probably a hydrated alkali metal hydroxide), similarly to what was observed in CaBi2 . Heat capacity measurements were performed using Quantum Design PPMS system by the standard relaxation technique. Magnetic susceptibility measurements were carried out using the ACMS option of the PPMS. Multiple randomly-oriented single crystals were put in the standard straw sample holders. Sample preparation was carried out inside a glovebox to prevent oxidation. Ab initio calculations were performed using the Quantum Espresso (QE) package employing ultrasoft Perdew-Burke-Ernzerhof (PBE) GGA 26 pseudopotentials 27 with valence configuration 4s 2 5s 1 4p 6 for Rubidium, 5s 2 6s 1 5p 6 for Cesium and 6s 2 6p 3 5d 10 for Bismuth, whereas the wavefunction and charge density energy cut-off's were set to values 50 Ry and 500 Ry, respectively. For the consideration of the influence of the relativistic effects, two types of calculations were performed: scalar-relativistic (neglecting SOC) and fully-relativistic (with SOC effect included for Bi). As expected and tested, inclusion of SOC for Rb and Cs does not change the results of calculations, thus the SOC was include only for Bi in final calculations. In the first step lattice constant was optimized using the Broyden-Fletcher-Shanno-Goldfarb (BFGS) method. The electronic structure was calculated at the grid of (24) 3 k-points. Phonon structure and properties of electron-phonon coupling (EPC) were calculated at grid of (6) 3 q-points. Chemical bonding was investigated by means of Crystal Orbital Hamilton Population analysis using the Stuttgart TB-LMTO-ASA code . Calculations were performed using the experimental cell parameter and (32) 3 k-point mesh and employed the Perdew-Wang GGA exchange-correlation (xc) potential. Electronic structure of an isolated tetrahedral Bi4 cluster was calculated using the ReSpect 5.1.0 relativistic DFT code . Scalar-relativistic calculations were performed by means of 1-component Kohn-Sham method with one-electron relativistic corrections due to Douglas-Kroll-Hess of second order (1c KS-DKH2). Fully relativistic electronic structure was calculated using the four-component Dirac-Kohn-Sham Hamiltonian (4c DKS). The Dyall core-valence double-zeta basis set and the PBE xc potential were used in both cases. Molecular orbitals obtained from the 1c KS-DKH2 calculations were visualized using the IboView program .