The following nuclear and molecular constants have been obtained from microwave measurements of the pure rotational spectra of $\mathrm{ClCN} (J=2\ensuremath{\rightarrow}3 \mathrm{transition})$, $\mathrm{BrCN} (J=3\ensuremath{\rightarrow}4)$, and $\mathrm{ICN} (J=4\ensuremath{\rightarrow}5)$. The moments of inertia from the ground vibrational states are, in ${\mathrm{g}\ensuremath{-}\mathrm{c}\mathrm{m}}^{2}$ \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}40}$: for ${\mathrm{Cl}}^{35}$${\mathrm{C}}^{12}$${\mathrm{N}}^{14}$, ${140.5}_{11}$; for ${\mathrm{Cl}}^{35}$${\mathrm{C}}^{13}$${\mathrm{N}}^{14}$, ${141.2}_{45}$; for ${\mathrm{Cl}}^{37}$${\mathrm{C}}^{12}$${\mathrm{N}}^{14}$, ${143.4}_{80}$; for ${\mathrm{Cl}}^{37}$${\mathrm{C}}^{13}$${\mathrm{N}}^{14}$, ${144.2}_{83}$; for ${\mathrm{Br}}^{79}$${\mathrm{C}}^{12}$${\mathrm{N}}^{14}$, ${203.6}_{21}$; for ${\mathrm{Br}}^{81}$${\mathrm{C}}^{12}$${\mathrm{N}}^{14}$, ${204.7}_{86}$; for ${\mathrm{Br}}^{79}$${\mathrm{C}}^{13}$${\mathrm{N}}^{14}$, ${205.9}_{64}$; for ${\mathrm{Br}}^{81}$${\mathrm{C}}^{13}$${\mathrm{N}}^{14}$, ${207.1}_{72}$; for ${\mathrm{I}}^{127}$${\mathrm{C}}^{12}$${\mathrm{N}}^{14}$, ${260.0}_{99}$; and for ${\mathrm{I}}^{127}$${\mathrm{C}}^{13}$${\mathrm{N}}^{14}$, ${264.0}_{72}$. The interatomic distances in A for the ground vibrational states are ${d}_{\mathrm{CCl}}=1.630$, ${d}_{\mathrm{CN}}=1.163$, for ClCN; ${d}_{\mathrm{CBr}}=1.789$, ${d}_{\mathrm{CN}}=1.160$, for BrCN; and ${d}_{\mathrm{CI}}=1.995$, ${d}_{\mathrm{CN}}=1.159$, for ICN. Quadrupole couplings in mc/s evaluated from the hyperfine structure are: for ${\mathrm{Cl}}^{35}$, -83.5; for ${\mathrm{Cl}}^{37}$, -65.0; for ${\mathrm{Br}}^{79}$, 686; for ${\mathrm{Br}}^{81}$, 573; and for ${\mathrm{I}}^{127}$, -2420.