The overall gas hold-up epsilon (g). the volumetric mass transfer coefficient k(1)a, the liquid side mass transfer coefficient k(1), the volumetric interfacial area a, the bubble size d(bs) and bubble distribution have been characterised in two bubble columns and a non-standard vessel equipped with various dual-impeller combinations. In the bubble columns, the effects of gas flow rate, sparger type and column diameter were investigated. In the dual-impeller stirred reactor, the effects of rotational speed, gas flow rate, impeller type and diameter were studied. The performances of the two reactors are compared here, and some relationships are proposed and compared with existing correlations.