A study is made of the effects of annealing both La2−xSrxCuO4 (for x = 0, 0.1, and 0.15) and YBa2Cu3O7 in wet and dry O2 atmospheres at various temperatures between 200 and 930 °C. In the case of La2−xSrxSrCuO4, substantial degradation of resistive properties occurs during annealing in moist O2, the degradation being highest at 200 °C and decreasing as the treatment temperature increases. Since the Meissner effect remains unaffected, it is concluded that degradation is due to the formation of a hydroxide species at grain boundaries, which decomposes as the anneal temperature is increased to 930 °C. In the case of YBa2Cu3O7, on the other hand, moisture does not produce degradation for anneals at 200 °C and above, but severe degradation of resistive behavior does occur for dry O2 anneals, with a maximum effect at 500 °C. It is found that this effect results from a contaminant gas, possibly CO2, absorbed by the furnace when it is open to air. Again, the degradation is due to formation of a grain-boundary phase which decomposes by annealing at 930 °C.