Cytochrome bo forms complexes with chloride, bromide and iodide in which haem o remains high-spin and in which the '630 nm' charge-transfer band is red-shifted by 7-8 nm. The chloride and bromide complexes each have a characteristic set of integer-spin EPR signals arising from spin coupling between haem o and CuB. The rate and extent of chloride binding decreases as the pH increases from 5.5 to 8.5. At pH 5.5 the dissociation constant for chloride is 2 mM and the first-order rate constant for dissociation is 2 x 10(-4) s-1. The order of rate of binding, and of affinity, at pH 5.5 is chloride (1) > bromide (0.3) >iodide (0.1). It is suggested that the halides bind in the binuclear site but, unlike fluoride, they are not direct ligands of the iron of haem o. In addition, both the stability of the halide complexes and the rate of halide binding seem to be increased by the co-binding of a proton.