Abstract The dependence of T c on oxygen content was determined for (Nd 0.67Ce 0.33) 2(Ba 0.67Nd 0.33) 2Cu 3.01O y . It was found that superconduction appears in a range of y=9.10−9.16 and that T c is enhanced remarkably by the increase of oxygen content. In an oxygen content range where superconduction appeared, the Ce valence was nearly +4 and the Cu valence changed from 2.31 to 2.33. The conductivity was measured in a wide temperature range between T c and 1200 K as a function of oxygen partial pressure. The high-temperature conductivity increased with increasing oxygen content. It was found that both carrier concentration and mobility are increased with oxygen content. The conduction at low temperatures was semiconductor-like, and increased with increasing oxygen content, where the energy gap E A- E V between acceptor level and top of filled band approached zero.