Abstract Superconducting properties of the YBaCuNbO system of nominal composition YBa 2Cu 3− z Nb z O x , with 0 ≤ z ≤ 0.1, are being investigated by XRD, and measurements of d.c. electrical resistivity and magnetic susceptibility as a function of temperature. XRD diffractograms showed the unambiguous existence of the YBa 2NbO y cubic perovskite phase, besides the YBa 2Cu 3O x (YBCO) orthorhombic perovskite phase, for z > 0.03. The superconducting zero resistivity temperature, T c0 , of the system increased slightly with the increase of z, reaching the maximum at z = 0.02. The temperature at the onset of Meissner effect, T s0 , also indicated a similar enhancement. The enhancement in T s0 from the ZFC data was higher than that from the FC data. The introduction of Nb from the starting oxide of Nb 2O 5 probably improved the oxygen content of the YBCO phase, and subsequently enhanced the superconducting transition temperatures.