Ammonia oxide (+NH3O-), a zwitterionic tautomer of hydroxylamine (NH2OH), has been proposed to explain the high reactivity of NH2OH with phosphate esters. The key parameter for kinetically significant reactions through +NH3O- is the value of the tautomeric constant K-T = [+NH3O-]/[NH2OH], which it has so far proved elusive to measure experimentally. Thermodynamic cycles using CBS-QB3 and CPCM computed data have been used to estimate microscopic and macroscopic acid-base equilibria involving NH2OH, the latter in close agreement to the experimental ones, but those involving anions. Calculated K-T is 2.6 x 10(-2), which indicates that although molecular NH2OH is favoured, there should be enough +NH3O- in aqueous solution to be a kinetically active species. (C) 2010 Elsevier B.V. All rights reserved.