In the case of a mixture of two isotopic molecules, $D$, the coefficient of ordinary diffusion, and $D_T$, the coefficient of thermal diffusion, are related by the equation: $\frac{D_T}{D}=\alpha C_1{C}_2$ where $C_1$ and $C_2$ are the relative concentrations of the molecules, and $\alpha$ is the thermal diffusion constant. Experimental values of $\alpha$ for neon and argon in seven different temperature intervals from 90-720°K are given. For both gases it appears that $\alpha$ varies linearly with the logarithm of the absolute temperature. These results are compared with values obtained from viscosity data and measurements made on binary mixtures of the noble gases. There is poor agreement with values calculated from the Sutherland and Lennard-Jones 9,5 models. In particular, no negative values of $\alpha$ are observed in the neighborhood of the critical temperature of argon as the 9,5 model predicts.

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