Abstract The reactions producing vibrationally excited CO in the continuous wave CS2 - O2 chemical laser are considered. Fundamental and overtone transitions were observed in the systems CS2 + O2, CS2 + N2O, O2 + CS and it is concluded that the two reactions O + CS → CO+S and O2 + CS → CO + SO are the reactions in which vibrationally excited CO are produced. The second reaction appears to be responsible for overtone laser action at low buffer gas partial pressures and low discharge currents.