The reactions of oxygen atoms with hydrogen cyanide, with cyanogen chloride and with cyanogen bromide, have been studied in a discharge flow system at temperatures between 450 and 650°K. The mechanisms are O + XCN = NCO + X, O + NCO = NO + CO, The following Arrhenius expressions were obtained for the initial steps. O + HCN, k= 5.2 × 1012 exp (–8,100 ± 1,200/RT) cm3 mole–1 sec–1, O + ClCN, k= 1.5 × 1012 exp (–6,900 ± 700/RT) cm3 mole–1 sec–1, O + BrCN, k= 1.4 × 1013 exp (–9,700 ± 1,200/RT) cm3 mole–1 sec–1.