The preparations and properties (H-1, C-13 NMR spectra; pK(a) data) of various bromoimidazole complexes of pentaamminecobalt(III) are reported. Improved methods are given for synthesising and isolating related methyl imidazole derivatives. Hydrolysis of [Co(NH3)5(tbim)]2+ (tbim = 2,4,5-tribromo-imidazolate) to give [Co(NH3)5(OH2)]3+ or [Co(NH3)5(OH)]2+ and 2,4,5-Htbim follows the rate equation K(obs) = (k1[H+] + k2K(a)[OH-])/(]H+ + K(a)) with k1 = 2.07 x 10(-4) s-1, k2 = 0.11 dm3 mol-1 s-1 and K(a) = 1.49 x 10(-2) mol dm-3 at 250-degrees-C and I = 1.0 mol dm-3 (NaClO4); k1 corresponds to the spontaneous hydrolysis of [Co(NH3)5(Htbim)]3+, and k2 to the OH--catalysed reaction of its conjugate base. Isomerisation of [Co(NH3)5(4-Hbim)]3+ (4-Hbim = 4-bromoimidazole) to its 5-bromoimidazole congener follows the rate equation (in D2O at 70-degrees-C) k(obs) = kK(a)[OD-]/(K(w) + K(a)[OD-]) with k = 1.67 x 10(-4) s-1 and pK(a) = 6.86; k corresponds to the spontaneous reaction of the deprotonated [Co(NH3)5(4-bim)]2+ ion and a pi-bonded, or solvent-cage-trapped, transition state is suggested. A crystal structure analysis of the dimer [(H3N)5Co(im)Co(NH3)5]Br5.3H2O (im = imidazolate) has been carried out: space group P2(1)/n, a = 12.946(4), b = 14.469(5), c = 14.022(4) angstrom, beta = 107.18(2)-degrees, Z = 4, 2191 reflections, R = 0.0576. Less accurate structures of four other complexes have been determined for the purpose of identifying the positions of bromine substitution.