Longterm damage mechanism investigations have identified effects responsible for laser induced damage. Particularly, the fundamental wavelength of Q-switched solid state lasers as well as their second and third harmonics have been analyzed in the context of the corresponding damage mechanisms. As a consequence of the immense progress in the production of coatings with lowest optical losses, the damage behavior of state of the art coating systems can typically be traced back to the contribution of microscopic defects and inclusions in the coatings for the VIS- and NIR-spectral range. In contrast, the influence of the intrinsic and surface absorptance can generally not be neglected in coatings for the DUV/VUV spectral region. This aspect gains of importance in the course of an increasing interest in the fourth harmonic for applications in research and industry. Therefore, the present paper is dedicated to investigations in oxide optical coatings for 266nm. This work has been performed to establish a database on the correlation of contamination and respective cleaning procedures to the damage threshold in the UV spectral region. Absorptance and degradation effects are identified by means of ISO certified laser calorimetry.