The aerated power consumption and the volumetric mass transfer coefficient were measured in a mixing vessel without baffles and with weak baffle conditions, equipped with dual impellers that combined a concave turbine and a Rushton turbine with a large ring sparger. When the ratio of sparger diameter to impeller diameter was >= 1.3, no decrease was observed in aerated mixing power consumption. It was shown that the volumetric mass transfer coefficient for a vessel without baffles and even with weak baffle conditions can be calculated by the correlation of Sato et al. (1989).