Oxygen diffusion in undoped and silver-doped (YBCO) single crystals has been investigated by isothermal resistivity measurements in the temperature interval . The diffusion coefficients for silver-doped crystals were found to be an order of magnitude lower compared with undoped ones. Further, the activation energy for oxygen diffusion was found to be considerably higher for Ag-doped crystals. Analysis of data on the equilibrium value of resistivity as a function of temperature and magnetization measurements made on samples quenched from an annealing temperature of , indicates that silver doping stabilizes the YBCO phase with higher oxygen content.