In this study, the mass transfer efficiencies of a novel horizontal rotating packed (h-RPB) bed and the conventional disc-type rotating biological contactor (RBC) were studied at four speeds and seven submergences. Pall rings of two different sizes (25, 38 mm), superintalox saddles and a wiremesh spiral bundle were used as packings in the h-RPB. Volumetric gas-liquid mass transfer coefficients were determined by unsteady state absorption of atmospheric oxygen in de-aerated water. Power consumption per unit liquid volume has been found for all geometries tested. The oxygen transfer efficiency values for the h-P.PB were found to be 2-5 kgkWh(-1) and for the disc RBC were found to be 1-2 kgkWh(-1). The performance of the h-RPB was also compared with other gas-liquid contactors such as surface aerators. The study proves that the h-RPB is a energy efficient alternative to conventional contactors. (c) 2005 Society of Chemical Industry.