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The ablation rates of a polyimide and a triazene polymer were studied gravimetrically by a quartz micro balance for 248-nm and 308-nm irradiation. Special care was taken to examine the dependence of the ablation rate at constant fluences for single pulses and the influence of consecutive pulses at the same position. A clear trend was observed in these measurements, i.e., that the mass loss after the first pulse is always different from values for the following pulses. This implies that it is very difficult to determine true ablation rates, which are the foundation of most ablation models. The differences of the mass loss between the first pulse and the following pulses is most probably due to carbonization of the material, resulting in varying ablation rates for the following pulses. The ablation rates are thus not a real material property but a superposition of the material ablation rates with the ablation rates of carbon and carbonized material. UV laser ablation of polymers is the objective of intense experimental and theoretical research due to the potential applications in many fields (chemistry, physics, biology, and electronics) [1,2]. Studies of the laser-induced decomposition or transformation of polymers with single pulses and laser fluences close to the threshold of ablation require very sensitive techniques such as a quartz crystal microbalance (QCM) and atomic force microscopy (AFM). One important question in polymer ablation is related to the ablation mechanisms, i.e., photochemical versus photothermal. The understanding of these mechanisms is important for the design of new materials (e.g., photosensitive polymers) or the optimization of existing industrial processes. Models play an important role in the understannding of these mechanisms. The mass loss during laser ablation is one important parameter for the development of theoretical models that decribe ablation.A mass loss (e.g., by QCM) due to photochemical reactions is expected for photosensitive polymers before a change

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