Transference numbers of aqueous hydrobromic acid have been determined from measurements of e.m.f.'s of concentration cells with and without transference, over a HBr concentration range up to 5.5 molal, at temperatures from 20°to 40°C.The DATA available in the literature for transference numbers of aqueous hydrobromic acid are very scanty and discrepant (1, 2, 6, 10, 17). Such a lack is surprising if one considers that, in the parallel case of hydrochloric acid, accurate and consistent data on transference numbers are available over a wide range of concentrations and temperatures (7,11).