The effect of relative humidity (0%, 40%, 51%, and 78%) on the stability of YBa2Cu3O7−xwas investigated over a six‐month period using x‐ray diffraction, scanning electron microscopy, and magnetic susceptibility measurements. At 78% humidity, the fraction of superconductive phase decreased parabolically with time. After six months, the powders completely decomposed into BaCO3, Y2BaCuO5, and CuO. Most of the samples placed at humidity levels of 51% or less were in the orthorhombic YBa2Cu3O7−xphase at the end of six months and remained superconducting and stable. However, all powders kept at nonzero humidity levels were heavily agglomerated; some became hard solids with diameters equal to or greater than 0.1 mm. The microstructure of the powder indicated a surface reaction with water undetected by x‐ray diffraction or magnetic susceptibility measurements.