FTIR smog chamber techniques were used in the first investigation of the products following oxidation of CH3C dropN initiated by Cl atom or OH radicals in air at 296 +/- 2 K. It is shown that reaction of Cl atoms with CH3C dropN gives (CH2C)-C-.=N radicals in essentially 100% yield. These radicals add O-2 to form the peroxy radical N drop CCH2O2.. Self-reaction of the peroxy radicals gives the alkoxy radical N drop CCH2O. which reacts exclusively with O-2 to give HC(O)C dropN (formyl cyanide). The reaction of Cl atoms with HC(O)C dropN proceeds with a rate constant of (7.0 +/- 1.0) x 10(-13) cm(3) molecule(-1) s(-1) via H-abstraction to give C-.(O)C dropN radicals which add O-2 to give N drop CC(O)OO. radicals. The subsequent chemistry of these radicals leads to the production of HC dropN and CO2 or the acyl peroxy nitrate N drop CC(O)OONO2, the IR spectrum of which is reported here. The reaction of OH radicals with CH3C dropN in 700 Torr of air gives HC(O)C dropN in (40 +/- 20)% yield implying that up to half of the OH + CH3C dropN reaction could proceed via H-atom abstraction, with the remaining fraction proceeding via addition to the -C dropN group. These results are discussed in terms of the atmospheric oxidation mechanism of CH3C dropN.