Emission spectroscopy was employed to study reaction process of poly(3-octyl thiophene) (P3OT) exposed to helium, nitrogen, air and hydrogen plasmas. The plasma was generated by an AC discharge at a pressure of 2 x 10(-2) Pa. The discharge power was maintained at an output of 220 V and a current of 0.2 A. The light emitted was monitored in the case of each plasma. Measurements at various discharge times were made of several emission peaks. The dominant emission bands were assigned to CH2 at 589.32 nm, S-2 (B-3 Sigma X--(u-)3 Sigma(-)(g) signed to CH2 at 589.32 nm, S-2 (13 " CH+ (A(1)Pi-X-1 Sigma) at 422.51 nm and C3H3 at 344.02 nm. These dominant emission bands indicate that the poly(3-octyl thiophene) was decomposed by plasma interaction. Nevertheless, the reaction between the polymer and nitrogen, hydrogen and air plasmas is more intense than that in the helium plasma. The possible reaction channels and formation mechanisms of some species produced are discussed. (C) 2007 Wiley Periodicals, Inc.