The 1H, 13C and 15N chemical shifts of benzotriazole and its adducts with acetone have been measured. Using dynamic 13C NMR spectroscopy in the +30/−90°C range, the barrier to prototropy in benzotriazole has been determined (10.8kcalmol−1 at 294K). The thermodynamic parameters corresponding to the equilibria between benzotriazole and its two acetone adducts have been measured. GIAO calculations (B3LYP/6-311++G∗∗) have been carried out on 1H-benzotriazole tautomer: they provide a sound basis for signal assignment of all the nuclei.