ABSTRACT In the past two years several laboratories around'the world have observed degradations of the Q-value of superconducting niobium cavities made from high purity niobium under certain cool-down conditions. Severe degradations have been reported for larger systems, when they were slowly cooled down or warmed up to temperatures A systematic study of the influence of surface treatments like chemical polishing, anodic oxidation and heat-treatment as well as the cooling conditions on cavity performance of cavities manufactured from niobium of different purity has been conducted. Possible reductions of the Q-degradations by anodic oxidation have been explored. Results of these investigations are reported and changes in material parameters as caused by the degradation are discussed