Y1Ba2Cu3O7−x superconducting thin films were grown in situ on polycrystalline MgO substrates by a chemical vapor deposition process that closely couples the sublimation temperatures of the elemental sources to that of the substrate during deposition. It was found that the best quality films were achieved with a controlled ramping of the substrate temperature from 850 °C, at the onset of deposition, down to 750 °C at the end of deposition. The films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM), x‐ray diffractometry (XRD), and four‐point resistivity probe. The results of these studies showed that the films were highly c‐axis oriented, had near‐stoichiometric composition, and exhibited Tc,onset=90 K, Tco=85 K, and had a Jc=2×105 A/cm2 at 77 K in zero magnetic field. A model is proposed for the effect of such temperature control on the CVD growth mechanism of high‐quality YBCO films.