Natural convection from a vertical electrically-heated plate (90 mm x 79 mm) to both Newtonian and non-Newtonian fluids has been studied experimentally for conditions of constant surface heat flux. For Newtonian fluids, a very wide range of viscosities (10 000-fold), of densities (1000-fold), and of Prandtl numbers (2500-fold) has been covered. The effect of shear-thinning non-Newtonian behaviour has been investigated using fluids with power-law indices from 0.48 to 0.81. Over this very wide range of conditions, the measured vertical temperature profile at the surface of the plate, and its dependence on the heat flux, has been found to conform well to theoretical predictions. The values of the coefficients in the well-established relations between Nusselt, Grashof and Prandtl numbers have then been evaluated and they have been shown to compare well with theoretical predictions and experimental results of other workers.