Fluorosulfonyl azide, FSO2N3, was characterized by IR (gas phase, Ar matrix), and Raman (liquid) spectroscopy. According to the matrix IR spectrum of O-18-labeled FSO2N3, its two oxygen atoms are nonequivalent. This assumption was confirmed by the X-ray crystal structure of FSO2N3 at - 123 degrees C, as only one conformer was observed with one of the S=O bonds in synperiplanar position to the N-3 group (phi(OS-NN) = -14.8(3)degrees) with respect to the S-N bond. The same conformation was found for trifluoromethylsulfonyl azide, CF3SO2N3 (phi(OS-NN) = -23.74(15)degrees), in the solid state. The preference of such a synperiplanar configuration between S=O and N-3 was rationalized by a predominant anomeric interaction of n(sigma)(N) -> sigma*(S-O), as supported by the experiment and quantum chemical calculations.