Abstract: Vertical gas holdup distributions in ambient gassed, hot sparged and boiling systems in stirred tanks using multiple 6 hollow blade disc turbines(CD-6)are reported.The local gas holdup in the boiling system is dramatically different from that in the ambient gassed or hot sparged system whether in terms of value or distribution.Under the same total gas flow rate and impeller speed,the local gas holdup is much smaller in the boiling system than that in the ambient gassed or hot sparged system.The hot sparged system has similar vertical gas holdup distributions,with maxima in similar locations but with smaller local gas holdup overall,to those of the ambient gassed system.The designs of existing hot sparged and boiling reactors are based on the ambient gassed system and not optimal.The results are of particular relevance to the design and operation of reactors with hot sparged or boiling liquids.