Ground-state rotational spectra of six symmetric-top isotopomers, C 6 H 6 H 7 9 Br, C 6 H 6 H 8 1 Br, C 6 D 6 H 7 9 Br, C 6 D 6 H 8 1 Br, C 6 H 6 D 7 9 Br and C 6 H 6 D 8 1 Br, of the benzene-hydrogen bromide complex were observed and analysed to give the rotational constant B 0 , the centrifugal distortion constants D J and D J K , and the Br nuclear quadrupole and spin-rotation coupling constants χ a a and M b b in each case. It is concluded that in the zero-point state the complex has effective C 6 v symmetry, with HBr oriented so that H lies closest to the benzene ring and undergoes a circular motion which allows it to sample the π-electron density of the ring.