As　an　alternative　disinfectant　to　free　chlorine,　monochloramine　reduces　the　formation　of　regulated　disinfection　byproducts　(DBPs);　however,　it　also　contributes　to　the　formation　of　highly　toxic　nitrogenous　DBPs　(N-DBPs),　especially　through　the　aldehyde　pathway.　The　current　understanding　of　aldehyde　pathway　mechanisms　is　limited.　In　this　study,　the　transformation　pathways　of　acetaldehyde　and　its　substituted　aldehydes　into　the　corresponding　nitriles　and　(N-chloro)amides　during　chloramination　were　investigated　using　quantum　chemical　calculations.　Consistent　with　previous　studies,　1-chloroamino　alcohol　first　forms　in　the　chloramination　of　aldehydes　and　then　undergoes　competitive　dehydration　and　HCl　elimination　branch　reactions　to　generate　the　nitrile　and　(N-chloro)amide,　respectively.　Iminol　was　found　to　be　a　key　intermediate　for　(N-chloro)amide　formation.　Moreover,　the　results　indicated　that　acetaldehydes　substituted　with　electron-donating　groups　(EDGs)　and　electron-withdrawing　groups　(EWGs)　are　beneficial　to　the　formation　of　the　respective　nitriles　and　N-chloro-amides,　while　those　substituted　with　conjugated　groups　(CGs)　are　favourable　for　both.　Based　upon　the　above　results,　in　addition　to　acetaldehyde,　other　aldehydes,　such　as　propionaldehyde,　glycolaldehyde,　3-butenal,　and　phenylacetaldehyde,　which　are　the　alpha-H　of　acetaldehydes　substituted　with　-CH3,　-OH,　-CH　=CH2,　and　-C6H5　groups,　respectively,　are　potential　precursors　of　toxic　nitriles　and　(N-chloro)　amides　during　chloramination.　Thus,　more　attention　should　be　given　to　these　aldehydes.　The　findings　of　this　work　are　helpful　for　further　understanding　the　aldehyde　pathway　mechanisms　and　predicting　potential　precursors　of　toxic　nitriles　and　(N-chloro)amides　during　chloramination.