This study made an attempt at applying ultrasonic technique to membrane-based deoxygenation process. The mass transfer performance of immersed hollow fiber membrane modules in removing dissolved oxygen from water with or without ultrasonic irradiation was evaluated. Moreover, to understand the influence of ultrasonic irradiation on the membrane, the surface structure of the irradiated membrane was observed by field emission scanning electron microscopy (FESEM). It was found that the mass transfer was greatly improved by ultrasound stimulation in both PP and PVDF membrane module. The mass transfer coefficient increased significantly with the increase of ultrasonic intensity, and up to 2.0 of the enhancement factor was obtained. In addition, the mass transfer improvement depended on the operating vacuum degree as well as the altitude of the immersed membrane module in the reservoir, due to the local difference in ultrasonic intensity. The microscopic images shed light on the changes of membrane structure under ultrasonic treatment. Either PP or PVDF membrane was affected by ultrasonic irradiation, the surface of PP membrane was more easily transformed and damaged than PVDF. In conclusion, ultrasonic irradiation can be applied to membrane-based deoxygenation for the enhancement of mass transfer, but cautions should be taken to choose proper membrane material, ultrasonic intensity and irradiated duration to avoid membrane damage. (C) 2007 Elsevier B.V. All rights reserved.