The potential of Meyer and Frommhold describing the interaction between hydrogen and helium atoms is used to calculate the diffusion coefficients of hydrogen atoms in helium gas and helium atoms in hydrogen gas as functions of temperature, taking into account higher-order corrections of transport theory. Comparison with experiment suggests that the interaction potential should be steepened at small nuclear separations. Calculations of the diffusion coefficients for temperatures between zero and 1000 K with the modified potential are reported. The corresponding H-He scattering length resulting from the zero-temperature value of the diffusion coefficients has the magnitude 0.359a 0 . Direct calculation shows it to be negative.