Laser ablation of Upilex-S polyimide films 80 mum thick was performed using a 355 nm pulsed Nd:YAG laser. Nanoparticles redeposited at the ablated zone and at the edge of the hole were observed. Chemical and structural changes of the Upilex-S polyimide surface after 355 nm laser irradiation in air were analyzed by X-ray photoelectron spectroscopy (XPS). Relative C content in the ablated zone was found to be higher, while both O and N contents were lower than in the untreated area. This indicates that laser irradiation released O and N atoms, and that the nanoparticles formed in the ablated zone consist mainly of carbon clusters. After deconvoluting the C 1s and O 1s spectra, three new components related to the carboxylate (O-C=O), ether (C-O), and dropC=O groups were detected in and around the ablated zone. In addition, one new component was found for O Is spectra, and the peak at 288.2 eV related to the imide carbonyl groups was shifted by 0.2-0.6 eV. These effects are probably due to the oxidation and formation of a non-imide carbonyl group. At the edge of the hole enhanced oxidation was found: the C content was considerably lower than that in the ablated zone, and the C 1s peak was at 288.6 +/- 0.2 eV. This suggests the formation of oxidized nano-particles, which carry the non-imide carbonyl groups. Based on these results, the decomposition of Upilex-S polyimide irradiated by UV light occurs via a photothermal ablation process.