Portion of the a-type J = 13–12 transition of HOCN observed in a discharge through water and cyanogen with a free-space millimeter-wave absorption spectrometer. Inserts show the K _a = 0 transition and K _a = 2 asymmetry doublet recorded with higher resolution and signal to noise on an expanded frequency scale. Integration times were approximately 5 min for each spectrum. The spectrum also shows the large density of unassigned background lines in our discharge source. The instrumental line shape, approximately the second derivative of a Lorentzian, results from the modulation scheme employed.