A solid state electrochemical technique is used to determine the oxygen chemical and component diffusion coefficients in the tetragonal phase of YBa2Cu3Ox. The measurements are performed at 850 degrees C, as a function of oxygen partial pressure and oxygen nonstoichiometry. The sample is held in a calcia-stabilized zirconia titration cell. An oxygen ionic current pulse is applied to the cell, and the EMF signal is monitored as a function of time. The diffusion coefficients are evaluated by analyzing the time dependence of the EMF, during and after the current pulse. It is found that the oxygen chemical diffusion coefficient increases weakly with x, with an average value of (D) over tilde similar to 5 . 10(-5) cm(2)/s at T = 850 degrees C. The oxygen component diffusion coefficient is found to be independent of nonstoichiometry, with a value of D-o similar to 5 . 10(-5) cm(2)/s.