Despite their drawbacks, chemical techniques are often the only means of determining volumetric gas-liquid mass-transfer coefficients k(L)a in gas-liquid and gas-liquid-solid reactors. Two techniques are considered here : the oxidation of sodium sulphite by oxygen in a viscous CMC solution and the oxidation of hydrazine catalysed by a copper ammonia complex. The latter technique is shown to be of use even when the liquid is not perfectly mixed. Furthermore, since hydrazine solutions exhibit coalescence, by adding sodium sulphate the coalescence behaviour of the liquid can be modified without affecting the kinetics of the reaction. In each case, the range of values of the mass-transfer coefficient k(L)a and the interfacial area allowing the use of this technique are given.