The nuclear quadrupole hyperfine structure of H 14 N 12 C and D 14 N 12 C has been resolved in the laboratory for the first time using millimeter-wave absorption spectroscopy. The transient species were produced in a pulsed DC discharge nozzle, and Doppler broadening effects were minimized by propagating the millimeter waves coaxially with the supersonic molecular beam. New rest frequencies for the , , and