Epitaxy of cupric oxides, such as the high temperature superconductor YBa2Cu3O7, using “vacuum” techniques requires either activated forms of oxygen, such as atomic oxygen, oxygen plasma, or ozone, or a relatively high pressure of molecular oxygen. In contrast, cuprous oxides (those with formal valence of copper less than +2) can be grown epitaxially in molecular oxygen at pressures below 10−4 Torr. We have explored this regime of epitaxial growth because of the possibility of forming DyBa2Cu3O7 through low temperature ex situ oxidation of DyBa2Cu3O6. We find that the dominant phases growing epitaxially on MgO are CuDyO2, Cu2O, CuBa2O2, DyBa2Cu3O6, and the barium-rich perovskite solid solutions. Sticking coefficients of barium and dysprosium depend on substrate temperature and flux composition for substrate temperatures between 550° and 700 °C. We have obtained superconducting films by annealing Dy-rich, Cu-deficient films in oxygen at 400 °C. The nonstoichiometry (with respect to DyBa2Cu3O6) appears to stabilize “DyBa2Cu3O6,” at low oxygen pressures. We also discuss the use of copper in effusion cells.