After observation of the degradation of bismuth cuprate superconductor by moistened air we have examined the action of water alone, through different techniques, in order to distinguish the proper role of water and carbon dioxide. In air the degradation of both powders and crystal occurs with the formation of alkaline earth carbonates, followed by that of hydroxides. Under the action of water vapor action, products are developed close to crystal defects and steps. Calcium hydroxide appears first and then strontium hydroxide, which undergoes a further hydration. The role of water seems to be two-fold : a chemical reagent and a physical medium allowing reactions to occur and crystal to grow. As for air action, the resulting compound of bismuth cuprate superconductor degradation is Bi2CuO4.