Abstract The dilatometric measurements for (CH 3 NH 3 ) 3 Sb 2 Br 9 and (CH 3 NH 3 ) 3 Bi 2 Br 9 crystals by means of a differential transformer between 90 and 220K were carried out. Both crystals exhibit a large anisotropy in the vicinity of all phase transitions in the linear thermal expansion measured along the c - and a -axis. The thermal properties of these two crystals were compared.