The radiofrequency plasma technique involving mixtures of CF4 + O2 gases has been applied to the surface treatment of high T(c) superconducting oxides (YBa2Cu3O7-delta). Investigation of the various experimental parameters of the process has shown that the improvement of the critical current density, J(c), mainly depends on the inlet precursor composition CF4 + tau%02, on the total pressure and on the reaction time. The presence of fluorine in the bulk of the ceramics has been observed from electron microprobe analysis, together with an increase of the "Cu3+" content. The plasma-enhanced fluorination (PEF) treatment improves the superconducting properties of the materials : both values of the resistivity in the normal state and of the superconducting transition width are reduced and the critical transition temperature is improved by about 1 K.