The molecular beam magnetic resonance method has been used to determine the sign (and the magnitude in those cases where it has not been previously measured) of the quadrupole interaction energy of the alkali nuclei in the homonuclear molecules, that of ${\mathrm{Na}}^{23}$ and ${\mathrm{Li}}^7$ in the alkali halides, and that of ${\mathrm{Cl}}^{35}$ and ${\mathrm{Cl}}^{37}$ in KCl. An obstacle is inserted into the path of the beam so that its edge coincides with the position of the undeflected beam. Molecules of either positive or negative total magnetic moment are then removed from the beam which arrives at the detector. Certain maxima in the nuclear resonance spectrum at high magnetic fields ($\mathrm{eqQ}\ll g_I{\mu }_{0}H$) arising from the transitions $\Delta m_I=\pm 1$ are suppressed depending on the states removed by the obstacle. It is thus possible to identify the resonance maxima in terms of the transitions which produce them. From this evidence the sign of the quadrupole interaction energy can be deduced. The quadrupole interaction energy, $\mathrm{eqQ}$, is positive for ${\mathrm{Li}}^7$ and negative for ${\mathrm{Na}}^{23}$ in the homonuclear and the halide molecules. These results suggest that the sign of $q$ at a given nucleus is the same in a rather considerable range of diatomic molecules. The interaction constant, $\mathrm{eqQ}$, is positive for ${\mathrm{Cs}}^{133}$ in ${\mathrm{Cs}}_2$, and negative for ${\mathrm{K}}^{39}$ in ${\mathrm{K}}_2$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Rb}}^{87}$ in ${\mathrm{Rb}}_2$, and ${\mathrm{Cl}}^{35}$, ${\mathrm{Cl}}^{37}$ in KCl. The signs of the interaction for ${\mathrm{K}}^{39}$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Rb}}^{87}$, and ${\mathrm{Cs}}^{133}$ in the alkali fluorides, and for ${\mathrm{Cl}}^{35}$, ${\mathrm{Cl}}^{37}$ in TlCl as determined by the molecular beam electrical resonance method are the same as those for the same nucleus in the molecules here considered.

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