In　vivo,　N-hydroxymethyl-methyinitrosamine　(HMMN)　and　N-acetoxymethyl-methyinitrosamine　(AMMN)　are　the　activated　derivatives　of　N-nitrosodimethylamine　(NDMA)　which　is　a　potent　carcinogen.　The　mechanistic　pathway　of　the　transformations　of　HMMN　and　AMMN　have　been　investigated　at　B3LYP/6-311+G(d,p)　level.　The　results　show　that　the　decomposition　of　HMMN　to　a　mono-function　alkylating　agent　proceeds　through　a　stepwise　mechanism　involving　isomerization　and　retro-ene　reaction,　in　which　the　former　is　the　rate-determining　step　in　the　gas　phase　with　the　energy　barrier　of　about　24　kcal/mol.　For　the　transformation　of　AMMN,　a　bi-function　alkylating　agent　is　produced　via　an　intramolecular　rearrangement　mechanism　which　is　similar　to　the　Favorskii　reaction.　(C)　2008　Elsevier　B.V.　All　rights　reserved.