In many applications of ArF - excimer lasers, a specific degradation effect is observed for the CaF2 outcoupling windows which starts assumedly at the rear surface and results in a characteristic damage morphology. In the present study, this degradation mechanism is examined in a measurement series involving a variety of window samples and irradiation sequences in an excimer laser with typical numbers of up to 2×108 pulses for each component. The irradiated samples were inspected by scanning spectrophotometry, TOF-SIMS, electron microscopy and other analytical techniques in order to clarify the underlying degradation mechanisms. On the basis of the experimental findings, coating strategies will be outlined to improve the lifetime of CaF2 - output couplers in 193nm excimer lasers.