Author Institution: National Bureau of Standards, Infrared \& Microwave Spectroscopy SectionThe Δ vibrational levels (2ν22​) of HCN and DCN are split by a rotational resonance with the nearby Σ vibrational level (2ν20​). At high J values the splitting becomes large enough so that ΔJ=0 transitions between Δ levels can be observed at microwave frequencies. Transitions for J=15 to J=23 have been observed in HCN and DCN. The agreement with calculated values will be discussed. New measurements on the usual l-doublet transitions will also be presented