The deposition of thin films by magnetron sputtering is readily influenced by the reduction/oxidation characteristics of the plasma. Unfortunately, the redox state of the sputter plasma is often inadvertently determined by uncontrolled factors such as transient evolution of species from the target. Undesirable variations in film and target properties can be avoided by actively setting the redox conditions. This is most readily accomplished by including two competing species, one oxidizing and the other reducing, in the chamber gas mixture. Optical emission spectroscopy, an in-situ process monitor, was employed to observe redox interactions in nominally unreactive dc sputtering of YBCO and reactive rf sputtering of IrO2. Optical spectroscopy of YBCO sputtering reveals that the intensity of atomic oxygen emissions decreases with H2 additions to the sputter gas blend.