Ab initio molecular orbital calculations including correlation interaction have been performed on formoyl, thioformoyl and acetyl azides using fairly large basis sets. The structural and energetic parameters were calculated at optimized geometry. Cyclic-open tautomerism has been theoretically investigated and the results showed that formoyl azide and acetyl azide are more stable than the corresponding cyclic conformers, oxatriazoles, whereas 1,3,3,4-thiatriazole is more stable than thioformoyl azide. The process of cyclic --> open conversion of the studied compounds was investigated theoretically and the energy of activation of such process was calculated.