[en] Magnetization studies were carried out on Y₁Ba₂Cu₃O_z (YBCO) thin films that were e-beam evaporated onto circular discs of single-crystal SrTiO₃ with (001) and (110) faces as well as KTaO₃ with (001) faces. The measurements were made using vibrating sample (VSM) and SQUID-based magnetometry with the applied field perpendicular to the substrate surface. Critical current densities J_c(H,T) are deduced from the magnetic hysteresis. Flux creep effects are observed over longer periods with the SQUID magnetometer. Analysis of the results of low frequency response of these films to collinear ac and dc magnetic fields are compared with the dc magnetometry results. J_c(H,T) is observed to be quite sensitive to the type of epitaxial growth on the various substrates. 16 refs., 10 figs