This paper considers the power demand of various types of turbine impellers operating with no sparged gas in boiling liquids, and how this differs from performance in dispersed gas-liquid systems. The results presented relate to Rushton, Hollow Blade and Pitched Blade Turbine impellers operating in boiling water in a vessel of 0.44 m diameter. Over operating ranges of practical interest, the relation (RPD) between two phase and single phase power demand for a given impeller can be expressed as : (RPD) = P(B)/P(U) = constant (2Sg/nu(t)2)0.4 In this expression P(B) and P(U) are the power demand under boiling and single phase conditions respectively, S is the submergence and nu(t) is the tip speed of the impeller. The constant depends on the impeller type; values, which are about unity, are given for the agitators used. The relationship, which provides a simple and adequate basis for design, is independent of the size of the impeller and the rate or location of vapour generation. Impellers continue to draw power, and presumably operate effectively, at vapour volumetric generation rates in excess of those for which sparged gas would swamp the pumping action. The results imply that in multiphase flows there may be large differences between Drag Coefficients for bodies in boiling systems and in gas-liquid mixtures of similar void fraction. The inherent differences between gas dispersing and vapour generating systems are discussed.