The relative stability of the NH 2 OH and NH 3 O isomers of hydroxylamine in aqueous solution was investigated at ab initio CCSD(T)/TZVPP+diff level for gas phase energy and Shells Theory of Solvation (STS) method for solvation contribution. Our calculations point out that the zwitterionic form (NH 3 O) is only 3.5kcalmol −1 less stable than the normal isomer (NH 2 OH). The solvation contribution obtained by the SM8 and SM8AD models are in close agreement with the STS calculations, whereas the PCM (or CPCM) and cluster-continuum methods need small cavities for predicting the correct stability value.