Deactivation of Hen egg white lysozyme and lipase from Aspergillus oryzae at the gas-liquid interface was comprehensively investigated in three different contactors, viz., bubble column, stirred vessel and falling film. Deactivation experiments were carried out over a wide range of power consumption (0.098 < (P-G/V) < 4.80 kW/m(3)), gas-liquid mass transfer coefficient (0.035 < k(L)a < 0.95 s(-1)) and gas-liquid interfacial area (20 < a < 346 m(2)/m(3)). A two-step series mechanism has been proposed and the major deactivation was found to be due to the first step. The rate was found to be first order and the rate constant was found to correlate well with the liquid side mass transfer coefficient. Further, the extent of enzyme deactivation was found to be depending upon gassed power consumption per unit volume. The structural change of deactivated lysozyme after exposure to air-water interface was investigated by fluorescence spectroscopy and circular dichroism spectroscopy. (C) 2003 Elsevier Ltd. All rights reserved.