Emission spectroscopy was applied to observe decomposed species of polyacrylonitrile (PAN) exposed with oxygen (O2) plasma, which was generated by microwave discharge with 8×10-2 dm3/s flow rate in the range of 1.8–4.7 Torr. As O2 plasma was exposed to the polymer, the surface was etched and a violet emission was observed on the surface. The strong emission assigned to CN(B2Σ–X2Π) transition appeared near 340–460 nm and also CH(A2Δ–X2Π) at 431 nm. These results indicated that the PAN sample was decomposed by the plasma etching and the decomposed species emitted the violet light on the surface. The time dependence of the emission intensities was also investigated. Wh n the discharge time of O2 plasma increased, the emission intensities of the CN and CH transitions increased and then gradually decreased. Evidence was presented by infrared measurements that a hetrocyclic nitrile group was formed on the treated surface. Furthermore, the polymer surface decomposition in O2 plasma was compared with that observed in nitrogen (N2) and air plasma. The N2 and air discharge showed less PAN decomposition than observed in O2 discharge. © 2001 American Vacuum Society.