Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient αeff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.