The molecular beam electrical resonance method, which has heretofore been confined to the study of transitions between the $m=0\mathrm{}$ and $m=\pm 1$ states of diatomic polar molecules in the state $J=1\mathrm{}$, has been extended to the study of rotational transitions from $J=0\mathrm{}$ to $J=1\mathrm{}$. The inhomogeneous electric fields together with a stop wire are so arranged that molecules in the state $J=0\mathrm{}$ are selectively refocused on the detector. A transition to the state $J=1\mathrm{}$, under the influence of the applied rf field, is observed by a reduction in beam intensity at the detector. The spectra obtained in this way yield more information than with the older method. The hyperfine structure of the state $J=1\mathrm{}$ of the molecules ${\mathrm{K}}^{39}$${\mathrm{Cl}}^{35}$, ${\mathrm{K}}^{39}$${\mathrm{Cl}}^{37}$, and ${\mathrm{K}}^{41}$${\mathrm{Cl}}^{35}$ has been investigated at zero field for several vibrational states. The molecular constants obtained for these molecules are: The structure of the $J=1\mathrm{}$ state was attributable to a nuclear quadrupole interaction for each nucleus. The variations of ${\mathrm{}\left(\mathrm{eqQ}\right)\mathrm{}}_{\mathrm{K}}$ and ${\mathrm{}\left(\mathrm{eqQ}\right)\mathrm{}}_{\mathrm{Cl}}$ with vibrational state are The ratios of the quadrupole moments of the potassium isotopes is $\frac{Q_{{\mathrm{K}}^{41}}}{Q_{{\mathrm{K}}^{39}}}=1.220\mathrm{}\pm \mathrm{0.002.}$ From observation of line shifts in a weak electric field, the molecular dipole moment was determined in two vibrational states. For $v=0\mathrm{}$, $\mu =10.48\mathrm{}\pm 0.05$ Debye, and for $v=2\mathrm{}$, $\mu =10.69\mathrm{}\pm 0.05$ Debye. The ratio of the dipole moments is 1.020±0.004. The mass ratios of the chlorine and potassium isotopes calculated from the molecular constants are $\frac{{\mathrm{Cl}}^{35}}{{\mathrm{Cl}}^{37}}=0.9459803\mathrm{}\pm 0.0000015 \mathrm{and} \frac{{\mathrm{K}}^{39}}{{\mathrm{K}}^{41}}=0.9512189\mathrm{}\pm \mathrm{0.0000015.}$

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