Room temperature PXRD patterns of crushed crystals are presented in Fig. of the Supplementary Material. All the Bragg lines in the PXRD pattern can be indexed to cubic (space group Fd-3m, #227) unit cell of ABi2 and elemental Bi (both from left-over flux droplets and decomposition of material). Lattice parameters resulting from LeBail fits are in agreement with previous reports . Details of crystal structures are gathered in Table 1. The Rb-Rb and Cs-Cs distances are 4.20 Å and 4.26 Å, significantly shorter than in their elemental form (4.89 Å and 5.29 Å, respectively ), while Bi-Bi spacing is larger than in elemental Bi (3.47 Å vs. 3.07 Å ) and in the Bi4 cluster (2.98 Å as obtained by ab initio structural relaxation). This highlights the electron transfer from the electropositive alkali metal to the Bi network. To characterize superconducting transitions of ABi2, dc magnetization was measured in the temperature interval T = 1.9 K -5 K. The plot of low-field (Hdc = 20 Oe) volume susceptibility (𝜒 𝑉 ) vs. temperature is shown in Fig. ). Sharp transition to a full Meissner state (𝜒 𝑉 reaching the value of -4π after correcting for the demagnetization effect) was observed in both materials. Very small difference between zero-field cooled (ZFC) and field-cooled (FC) magnetization (not shown) is indicative of weak (or nonexistent) vortex pinning. Field-dependent magnetization below the critical temperature is shown in Fig. ). In case of RbBi2 the shape of magnetization curve is consistent with a type-I superconducting transition with an influence of the demagnetization effect due to randomly-oriented crystalline grains . In CsBi2 the shape of the M vs. H curve has a tail-like feature at higher fields, which is more consistent with type-II SC. Assuming a perfect diamagnetic linear response to the magnetic field, the Mv vs H data at the lowest temperature (T = 1. showing an anomaly at the superconducting critical temperature (enlarged in insets). The normalized heat capacity jump ΔCp/γTc is calculated using γ values estimated from a fit presented in Fig. of the Supplementary Material. Note that in both cases the jump under an applied magnetic field is found to be larger than at zero field, which is a typical behavior for type-I superconductors (see eg. refs. ).