A new oxidation technique of high temperature superconducting copper oxides has been invented. The oxidation is performed in a low., pressure oxygen plasma generated by a microwave. X-ray and the low temperature alternating current magnetic susceptibility have been used to explore the effect of the oxidation on the superconducting behavior. Also, the measured infrared spectra reveal good dependence of the two copper-oxygen populations. (C) 2002 Elsevier Science B,V All rights reserved.