Rotational spectra for formyl azide (HC(O)N3) were obtained from 240 to 360 GHz. 1573 rotational transitions were observed for the ground state of syn formyl azide. Six vibrational satellites of syn formyl azide were observed. Excellent agreement: measured, computed vibration-rotation interaction. Thermal decomposition kinetics of formyl azide monitored (half-life t½ = 30 min).