The geometries of various tautomers and isomers of 2-amino-2-imidazoline, 2-amino-2-oxazoline and 2-amino-2-thiazoline have been studied using the Becke3LYP/6-311+G(d.p) DFT, MP2/6-311+G(d,p), and CBS-Q model chemistries. At each of the foregoing levels of theory, the amino tautomer is computed to be more stable than the imino tautomer. The iminozation free energies were found to be 5-14 kJ/mol, The computed imine <----> amine tautomer equilibrium onstant (K-T much greater than 1) increases in the order: amino derivative < sulphur derivative < oxygen derivative, Solvation (using the SCRF formalism) causes a shift in tautomeric preference towards the imino species. Of the two possible (E and Z) isomers of 2-imino-2-oxazolidine and 2-imino-2-thiazolidine the (Z)-isomers have the lowest energy, The most stable structures are stabilised via intramolecular hydrogen bonds.