The synthesis of "carbon-free" YBa2Cu3O7-x-123 by metal oxidation is reported for normal oxygen (O-16) and isotope O-18 containing samples. This method allows a maximum isotope content of 97% if O-18(2), of 99% is used. The oxygen isotope ratio is determined both by thermogravimetry and SIMS. It is shown that the thermogravimetric determination is very accurate in this case (+/-2%). The phase purity of the samples has been shown to influence the thermogravimetry measurements (TG).