The mass transfer in the air-water system was investigated for various single- and twin-impeller systems including the standard Rushton impellers, pitched-blade impellers, concave-blade impellers and comb-blade impellers. For twin-impeller systems, the effects of the two impellers operating at various speed combinations, opposite impeller rotation and alternate impeller rotation were investigated. Results showed that although slightly better performance was achieved by using the concave-blade and comb-blade impellers, all the volumetric mass transfer coefficients, k(L)a, obtained were mainly determined by the specific power consumption (P/V) and the superficial gas velocity (U-s) and correlated reasonably well by a single correlation due to van＇t Riet. The possible reason for the better performance of the comb-blade impellers is discussed.