Oxygen mobility in the system YBa2Cu307-δ (0 ≤ 8 ≤ 1) has been studied by electrochemical methods over the temperature range 500 ≤ T ≤ 800 °C. The experimental cell consisted of a pellet of YBa2Cu307-δ sandwiched between an ionically-blocking Au film electrode and an electronically-blocking yttrium-stabilized zirconia electrolyte/Pt paste electrode. The concentration of mobile oxygen ions, c(02-), and the diffusion coefficient, D, were evaluated from potential step experiments. Coulometric titration studies were used to modify the oxygen content of the oxygen deficient (δ ≥ 0) material. These electrochemically-modified samples were further characterized by magnetic susceptibility measurements to monitor the effect of changes of oxygen content on the width and position of the superconducting transītion.