To determine the oxygen mass transfer in clean water in biofilters, a method based on the follow up of the oxygen fraction in the off gas during the oxidation of sulphite in excess has been evaluated and applied to a pilot-scale unit (250 L, superficial gas velocity from 0 to 5.6 x 10(-3) m(N) s(-1), superficial liquid velocity from 0 to 2.5 x 10(-3) m s(-1)). Tests performed on a two-phase reactor showed that, without any cobalt addition, standard oxygen transfer efficiencies (SOTE) obtained from the proposed method are not statistically different from those issued from the standardised method. A relationship has been proposed to express SOTE values as a function of the conductivity, and the influence of the gas and liquid velocities on SOTE and k(L)a has been investigated. (c) 2005 Elsevier Ltd. All rights reserved.