High temperature superconductor YBa2CU3O7-x thin films have been grown using in situ electron beam evaporation at deposition rates over 1-10 nm/s. The higher activity of atomic oxygen is being explored along with molecular oxygen at reduced pressure (5 x 10(-5) Torr). Preliminary observations indicate that YBa2CU3O7-x is Stable within a band of atomic oxygen flux and temperature, similar to that published for YBa2Cu3O7-x in molecular oxygen but many orders of magnitude lower in flux. The phase equilibrium appears to be different from that found for molecular oxygen at low rates.