YBa2Cu3O7−xsuperconducting thin films prepared by chemical vapor deposition which exhibit high transition temperatures (Tc∼90 K) and high critical current densities (Jc≳106A/cm2at 77.7 K and 0 T) generally have copper‐rich precipitates on the surface. We have studied both near‐stoichiometric and nonstoichiometric highlyc‐axis oriented thin films formed by plasma‐enhanced metalorganic chemical vapor deposition. We show that the reduction in transport properties (TcandJc) observed in stoichiometric films with smooth morphologies may result from a dramatic reduction of the oxygen diffusion rate in these thin films as compared to nonstoichiometric films. The significant enhancement of the transport properties of these films was achieved by further oxygen anneals at 480 °C.