The phase transitions of Cs3Sb2I9 , Cs3Bi2I9 , and Cs3Bi2Br9crystals were investigated by 133Cs nuclear magnetic resonance. Cs3Bi2Br9was also studied by 87Br nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs3Sb2I9 , the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs3Sb2I9has a trigonal structure. The observed temperature variation in the shape of the 133Cs NMR and 127I NQR signals suggests that Cs3Bi2I9may undergo a transition to an incommensurate phase.