An expression for the energy exchange between two Maxwellian gases at different temperatures is derived. The resultant formula contains an integral in which the scattering cross section for momentum transfer appears. If the cross section for hard‐sphere collisions is used, the expression reduces to an expression obtained by Cravath for this case. If the cross section for screened Coulomb interactions is used, the results agree with results for the same case obtained by Spitzer and others.