This work reports the effects of the presence of a gas or vapor phase on mechanically agitated liquid mixing. A conductivity technique was used to follow the assimilation of tracer added at the liquid surface of cool aerated, hot sparged, and unsparged boiling liquid. The 75-L-capacity baffled vessel had a single, centrally mounted, Chemineer CD-6 hollow blade impeller. Sparging increases the rate of mixing at a given shaft power, especially at high gas flow rates: Inconsistencies in the literature on the effect of aeration on the mixing time are satisfactorily resolved if the potential energy input from gas sparging is considered. When this is done, hot and cold sparged and unsparged mixing times are comparable, provided that the contribution of the volume of any vapor generated is included. Mixing of surface added tracer into boiling liquid is considerably faster at a given impeller power than in any of the other conditions. This is probably a result of the additional disturbance at the liquid surface produced by vapor evolution causing a rapid initial distribution of tracer.