AbstractThe overall apparent volumetric gas—liquid mass transfer coefficient (k95a) and the mixing time (t95) were determined in a 240 dm3 vortex aerated fermenter over stirrer speed and air flow ranges of 300–800 rpm and 10–45 normal dm3 min−1, respectively. The mass transfer data obtained in an aqueous salt solution (2.5 kg m−3 NaCl in water) compared well with the measurements in a fermentation medium used in culture of certain microaerophilic bacteria. Over the ranges examined, the gas‐liquid mass transfer coefficient depended only on air flow rate; the dependence was linear with flow. Mixing time declined with increasing agitation according to a power‐law relationship. The mixing and mass transfer characteristics of the vortex aerated system were compared with that of a ‘standard’ stirred tank fermenter (27 dm3). The mixing time variations with respect to agitation rate were remarkably similar for the two types of fermenters examined.