The Mo¨ssbauer effect of the 37.2 keV &ggr; transition of121Sb has been employed to study the electronic configuration of Sb ions in BaBi1−xSbxO3withx=0.2, 0.4, and 0.5. A single resonance absorption line is observed. The isomer shift changes slightly with Sb content and falls in the region corresponding to an Sb(V) valence state. The isomer shift of 7.6 mm/s is significantly depressed (∼1 mm/s) relative to its value in a typical Sb(V)O6chromophore and indicates a high degree of covalency, corresponding to significant 5sm5pnhybridization.