Gas holdup measurements in cold gassed, hot sparged and boiling systems using different agitators with multiple modem hollow blade and up-pumping wide-blade hydrofoil impellers are reported. In a cold gassed system with given gas flow rates and power input, the different multiple impeller configurations generate very similar gas holdup. The gas holdup in a hot sparged system, between 70degreesC and 90degreesC, is only about half that in a cold system with the same total gas flow rate and power input. Moreover, at the higher temperatures there are unexpected and very significant differences between the gas holdup achieved by the different agitators. With most vapour generation occurring near the liquid surface, the vapour holdup in an unsparged boiling system is much lower than that for either cold gassed or hot sparged systems with similar total gas flow rate and power input. These results are of particular relevance to the design and operation of reactors with hot sparging or boiling liquids.