Eight　isomers　of　N2O3,　including　five　new　conformers,　were　investigated　at　the　CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p)　level.　The　computational　results　showed　that　asym-N2O3,　sym-N2O3,　and　trans-cis　N2O3　are　stable　isomers　and　asym-N2O3　is　the　most　stable.　The　five　new　isomers　are　much　more　energetic　than　asym-N2O3　by　75-82　kcal/mol.　All　eight　isomers　are　able　to　convert　among　each　other　through　one-step　or　multi-step　reactions.　Additionally,　the　N-nitrosation　of　dimethylamine　(DMA)　by　the　three　most　stable　N2O3　isomers,　i.e.,　asym-N2O3,　sym-N2O3,　and　trans-cis　N2O3,　Was　studied　at　the　same　theoretical　level.　Results　indicated　that　the　N-nitrosation　of　DMA　by　these　three　N2O3　isomers　should　easily　occur,　not　only　in　the　gas　phase　but　also　in　water.　Moreover,　the　energy　barriers　of　these　N-nitrosation　reactions　demonstrated　that　the　values　of　sym-N2O3　and　trans-cis　N2O3　isomers　are　very　close　or　even　lower　than　that　of　asym-N2O3　isomer.　Therefore,　the　results　obtained　in　this　work　lead　to　the　conclusion　that　along　with　the　known　asym-N2O3,　being　the　nitrosating　conformation　for　N2O3,　the　sym-N2O3　and　trans-cis　N2O3　isomers　also　have　relatively　strong　nitrosating　abilities.　This　conclusion　is　consistent　with　experimental　results　and　will　be　helpful　in　better　understanding　the　mechanism　of　N-nitrosation　of　amines　by　N2O3.　(C)　2009　Elsevier　B.V.　All　rights　reserved.