The average Nusselt number for the flow of a mixture of air and vapor across a tube was measured at moderate Reynolds numbers (2000-7000) for temperatures from 300 to 700degreesC and for humidity ratios from 0.22-0.54. Results are also presented for flow over a set of three tubes aligned perpendicular to the flow for the same range of conditions. The effects of humidity ratio and temperature on the convection coefficients were investigated to develop a modified Nusselt number correlation describing the heat transfer over the tubes. The results showed that the Nusselt number increased as the humidity ratio increased and that the increase was more than could be accounted for by typical models for the property variations of mixtures. The Nusselt number data were correlated with an equation that included both a Prandtl number ratio and a thermal conductivity ratio to correlate the multiple property variations due to both the large temperature variations and the large humidity variations. The exponent of the thermal conductivity ratio used in the modified correlation to correlate the additional effects of the property variations due to the large humidity variations was 0.073 for the entire data set, indicating the importance of the property variation due to the moisture content.