In addition to YBa2Cu3O7-x, material of the class RBa2Cu3O7-x, where R is a lanthanide element other than lanthanum, cerium, praseodymium, promethium or terbium, is also satisfactory, as are the superconductive bismuth-strontium-calcium-copper-oxygen compounds and the superconductive thallium-barium-calcium-copper-oxygen compounds.