Polycrystalline samples of the high-transition temperature superconductors Y–Ba–Cu, Ho–Ba–Cu and Gd–Ba–Cu oxides are characterized by resistivity, susceptibility, and x-ray measurements. Measured magnetic moments are ascribed mostly to magnetic rare-earth ions, and partly to Cu2+ ions and some other crystal phases. It is found that an oxygen pressure and a sample-preparation temperature dominate the amount of the Cu2+ ions and of other phases, respectively.