Degradation of the 90 K superconductor $YBa_2Cu_3O_{7-{delta}}$ in water and humid atmosphere were studied and its decomposition products were identified as $BaCO_3$, CuO, $Y_2(CO_3)_3{cdot}3H_2O$ and $O_2$. XRD analysis, iodometric titration and IR-spectroscopy of the degraded samples suggest that the $YBa_2Cu_3O_{7-{delta}}$ decomposes in three steps such as the reduction of $Cu^{3+}$ to $Cu^{2+}$, followed by the hydroxylation of $Ba^{2+}$ and $Y^{3+}$ via hydration and finally the formation of carbonates through the uptake of ambient $CO_2$.