The dynamic behavior of a photoetching process of a photosensitive triazenopolymer film during and after XeF excimer laser irradiation was studied by nanosecond interferometry, accompanied with time-resolved transmission and reflectance measurements, A new optical setup for nanosecond interferometry, which was free from the disturbance of ejected products, was developed and applied. At a fluence of 250 mJ/cm(2) a slight swelling of the film and darkening of the irradiated surface was initially observed, and then the etching process of the film was brought out around the peak time of the excitation laser pulse. The etching developed during the excitation laser pulse and stopped almost at the end of the excimer laser pulse. On the other hand, at a fluence of 60 mJ/cm(2), the etching started from nearly the end of the excimer laser pulse and persisted for 50 ns. The fluence dependent etching behavior was interpreted in terms of coupled photochemical and thermal decomposition processes of the triazenopolymer.