Interfacial area was measured in a helicoidal reactor for the two-phase gas-liquid flow. Measurements were accomplished using the method of oxygen chemical absorption in alkaline solutions of Na2S2O4. The process parameters were varied within the following ranges: rotational speed of impeller from 0 to 2900 rpm; liquid mass flux from 2.7 to 100 kg/(m(2)s); volumetric flow ratio of gas and liquid from 0.125 to 1; ratio of impeller diameter to the outer reactor diameter from 0.54 to 0.92. Large values of the interfacial area were found. It was established that they depend mainly on the rotational speed of the rotor and to much less extent on the axial flow velocity. A correlation between changes in the interfacial area and changes in the flow structure within the reactor was also observed. (C) 2004 Elsevier B.V. All rights reserved.