Neutron diffraction / Crystal structure / Hydrogen bromide / Molecular disorderSummary. Solid HBr exhibits a series of phase transitions from cubic la via lb, to orthorhombic II and III. The structure of lb has been solved and refined with neutron diffraction data. It crystallizes in space group Pai. The molecules show dynamical disorder, the hydrogen atom preferentially reorienting in planies formed by 6 out of 12 nearest neighbour bromine atoms. The phase lb does not occur with DBr but can still be observed with a 90% DBr/HBr sample due to the delayed formation of phase II with decreasing temperature. -The group-subgroup relations for the different HBr modifications are established.