The relative abundances of ${\mathrm{He}}^3$ in the liquid and vapor phases of atmospheric helium have been studied as a function of temperature above and below the $\lambda$-point. The relative abundance of ${\mathrm{He}}^3$/${\mathrm{He}}^4$ in the vapor ($C_v$) was determined by means of the mass spectrometer and the corresponding quantity for the liquid ($C_L$) computed from $C_v$ and the mass balance and composition balance equations. From 5.2°K to the $\lambda$-point the ratio $\frac{C_v}{C_L}$ increases as the temperature decreases, whereas below the $\lambda$-point this ratio sharply decreases to zero, showing that the vapor in equilibrium with superfluid helium contains no measurable amount of ${\mathrm{He}}^3$. On the basis of these results a simple method of concentrating ${\mathrm{He}}^3$ is suggested.

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