[en] We present upper critical field measurements on the superconducting oxides RE Ba₂Cu₃O_7-δ (RE = Y, Gd) performed in a pulsed magnetic field up to 43 T. Values as high as 52 T and 77 T for Y and Gd respectively, are expected at 77 K. However, in order to observe no resistive behaviour up to 43 T the temperature must be decreased down to 50 K. In the case of oxygen deficient systems the magnetoresistance reveals two superconducting phases which could be related to two different orders of oxygen vacancies