This paper considers the operation of gas-liquid reactors in which there is a significant evolution of vapour. The power demand characteristics of sparged agitators operating in hot liquids are considered in terms of the very different behaviour in the limiting conditions of dispersing gas in cold liquid or during unsparged operation in boiling liquids. A satisfactory basis for design is provided in terms of the limiting impeller power draw at flooding for the particular total loading of the agitator the sparged gas when saturated with vapour at the temperature of the bulk liquid.