Superconducting YBa2Cu3O7-x thin films were prepared on pure MgO and Ag/MgO substrates (without and with Ag buffer layer) using an electron beam evaporation technique. The effects of isothermal annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties were investigated by X-ray diffraction, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. The optimum annealing conditions causing a high degree of preferential orientation with the c-axis perpendicular to the substrates were found to be the isothermal annealing at 930 degreesC for 5 h. Annealing of films on Ag/MgO substrates is accompanied by Ag diffusion from the buffer layer into YBCO films. The higher rate of crystallization of the YBCO films, the higher degree of c-axis orientation, the higher dense surface morphology, the increased lattice parameter c (by approximate to0.1%), the reduced room temperature resistivity (2-3 times), the slightly enchanced critical temperature (T-c = 92 K at R = 0) and the critical current density (J(c) = 4.2 x 10(5) A/cm(2) at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates) in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in YBCO films in the range 600-800 degreesC was described by the relation D = 1.9 X 10(-6) exp(-0.73 eV/kT). (C) 2004 Elsevier B.V. All rights reserved.