The determination of k(L) a by a gas balance method coupled with sulphite oxidation is compared for three kinds of processes (stirred tank, bubble column and fixed-bed column reactors) with a gassing-in and with a classical chemical sulphite oxidation method. The mathematical relations required for the determination of the k(L) a value are detailed. In coalescing gas-liquid conditions, the values calculated by the three methods are shown to be comparable. The gas balance method is more rapid than either the steady-state gassing-in or the chemical sulphite reaction rate measurement methods. It is also well adapted for three-phase systems (gas-liquid-solid) in which the non-coalescing effects of sulphite solution are reduced by solid interferences.