Abstract The new compound Rb2SnO2 was obtained as a light yellow powder by heating mixtures of the binary oxides (RbO0.48 + SnO, Rb: Sn = 2:1) under argon (sealed Ag cylinders, 600 °C, 3d). Subsequent annealing (550 °C, 8d and 500 °C, 17 d) led to light yellow transparent single crystals. Rb2SnO2 is extremely sensitive to moisture and decomposes into black products. It crystallizes orthorhombicly in the space group P212121-D4 2, with a = 5.761, b = 7.493, c= 11.167 Å, Z = 4, drö = 4.43 and dpyk = 4.46 gcm-3 , R= 11.8 and Rw = 12.0%. Effective Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, and the Madelung Part of Lattice Energy, MAPLE, have been calculated. Isotypic K2SnO2 was obtained similarly as a brown powder, a = 5.579, b = 7.246, c = 16.744 Å.