On lowering the chemical potential of diatomic oxygen, the compound YBa2Cu4O8was found to decompose into a mixture of YBa2Cu3O6+xand CuO. The equilibrium oxygen potential corresponding to this decomposition has been measured from 870 to 1150 K using a solid‐state cell incorporating yttria‐stabilized zirconia as the solid electrolyte and pure oxygen at a pressure of 1.01×105Pa as the reference electrode: Pt, O’2‖YBa2Cu4O8+YBa2Cu3O6+x+CuO∥(Y2O3) ZrO2∥O2(1.01×105 Pa), Pt. The low oxygen potential boundary for the stability of YBa2Cu4O8can be represented by &Dgr;&mgr;O2=−238 900+207.7 T(±800) J mol−1. At the standard pressure of oxygen (1.01×105Pa) the decomposition occurs at 1150 K.