Abstrae~The purpose of this paper was to demonstrate, quantitatively, that certain gas mixtures are much more efficient in forced convective heat transfer applications than pure gases alone. This work was also deliberately limited to comparisons involving the relative heat transfer capability of selected binary gaseous mixtures and either pure helium (generally considered to be the second best gaseous heat transfer medium) or pure hydrogen (generally considered to be the best gaseous heat transfer medium) in the turbulent flow regime. The specific gaseous mixtures considered herein were: (1) helium plus one other noble gas and (2) hydrogen plus one noble gas. Corroborating data, with respect to the noble gas mixture data and pure helium, was also found (in an earlier but slightly different theoretical study) and discussed within this work. However, no corroborating data, with respect to the noble gas and hydrogen mixture data, could be found. In any case, all of the results presented in this paper are believed to be useful in optimizing the design and operation of gaseous heat exchangers that must rely upon either very compact design features or extremely efficient operation. '~, 1997 Elsevier Science Ltd. Keywords~Heat, transfer, binary, gas, mixtures, helium, hydrogen. NOMENCLATURE fl density of fluid Cp heat capacity of fluid at constant pressure D tube diameter hc film coefficient--forced convection k thermal conductivity of fluid mavg average molecular weight of gaseous mixture n viscosity of gaseous mixture P absolute pressure PR Prandtl number of fluid T absolute temperature V fluid velocity Yi mole fraction of ith gaseous component in gaseous mixture