A study of the microstructure of Cu-rich and stoichiometric Y-Ba-Cu oxide thin-film superconductors is presented. The films were deposited on <100> SrTiO3 by the nonvacuum technique of metalorganic deposition followed by rapid thermal annealing in oxygen. Analysis showed that for annealing temperatures below 900 °C, grain size increased with increased annealing temperature, with an enhancement in grain growth for the Cu-rich films. Annealing near or above the melting point of the 1-2-3 phase causes only a slight increase in the rate of grain growth and no detectable effects of the excess Cu. Annealing above 920 °C produces segregated CuO islands 5–10 μm in size in the Cu-rich films. Oriented grain growth was found for the 1-2-3 grains with their c axis perpendicular and parallel to the SrTiO3 substrates. Sheet resistivity measurements were correlated with grain size, phase separation, and oriented grain growth. An anomalous behavior in the resistance-temperature plot at 220–240 K of the Cu-rich films is shown to be related to the presence of the excess Cu. [ABSTRACT FROM AUTHOR]