Conducting layers on KrF excimer-laser-irradiated polyimide film surfaces were investigated by XPS, SEM and Fourier transform infrared (FTIR)– Raman spectroscopy. Analysis of polyimide residue after laser irradiation provided valuable insight into the nature of the formation of conducting layers. The subtle different between KrF laser irradiation and the pyrolysis of polyimide was found by comparison of the formation process of conducting layers. A physical picture was presented to describe better the formation of conducting layers. Under KrF laser irradiation, polyimide films underwent thermal decomposition assisted by photoinduced direct bond breaking. Polycrystalline graphite was subsequently formed as the product of the secondary addition reaction of carbon-enriched clusters. Such reaction was supported by the remaining energy on the irradiated polyimide film surface. This result shows that the thermal process played an important role that was not just restricted to the formation of conducting layers. Copyright © 2000 John Wiley & Sons, Ltd.