Tne effects of property variation on heat transfer to supercritical pressure fluids are reviewed, An experiment with supercritical pressure carbon dioxide is described for vertical flow in tubes of 5, 8 and 19mm diameter. Criteria are presented for the absence of buoyancy effects for upward and downward fLlow, and the published correlations for buoyancy free heat transfer to supercritical pressure fluids are critically examined, using the published data. Mixed convection correlations are presented for upward and downward flow in the form NU/Nu force = O (Gr/Reb 2.7) An experiment with atmospheric pressure water flowing in a vertical tube of 100mm diameter shows that the buoyancy correlations perform satisfactorily for mixed convection in fluids vrithout severe property variations.