Isolated nitrosoguanidine tautomers have been subjected to 6-31G(d,p), 6-31G(d,p)/(MP2), B3LYP/6-31IG(d,p) and B3LYP/6-311++G(d,p) type quantum chemical analyses in the gas phase. The geometrical features and energetics of some conformers of the tautomers are reported. The nitrosimine form has the highest stability than the others and the diazoic acid form is the least stable one. The nitrosoinfine form has the highest HOMO and LUMO energies. Whereas, the nitrosamine form possesses the lowest HOMO and the diazoic acid form has the lowest LUMO energies. (c) 2006 Elsevier B.V. All rights reserved.