An atmospheric-pressure plasma jet has been used to etch polyimide films at 1.0-8.0 +/- 0.2 mu m/min at 760 Torr and between 50 and 250 degrees C. The plasma was produced by flowing helium and oxygen between two concentric electrodes, with the inner one coupled to 13.56 MHz rf power and the outer one grounded. The etch rate increased with the O-2 partial pressure, the rf power and the substrate temperature. The apparent activation energy for etching was 0.16 eV. Langmuir-probe measurements revealed that the ion densities were between 1 x 10(10) and 1 x 10(11) cm(-3), 5 mm from the end of the powered electrode. Biasing the substrate had no effect on the rate. Ozone, singlet sigma metastable oxygen (b (1)Sigma(g)(+)), arid singlet delta metastable oxygen (a (1)Delta(g)) were detected in the plasma by emission spectroscopy. More ozone was produced in the effluent through the recombination of O atoms with O-2. Based on the production rate. of O-3, the concentration of O atoms 6 mm from the powered electrode was estimated to be similar to 7 x 10(14) cm(-3) at 6.6 Torr O-2 and 200 W power. It is proposed that O atoms are the principal reactive species involved in etching polyimide.