While the properties of functional oxide thin films often depend strongly on oxygen stoichiometry, there have been few means available for its control in a reliable and in situ fashion. This work describes the use of DC bias as a means of systematically controlling the stoichiometry of oxide thin films deposited onto yttria-stabilized zirconia substrates. Impedance spectroscopy is performed on the electrochemical cell Pr0.1Ce0.9O2-delta(PCO)/YSZ/Ag for conditions: T = 550 to 700 degrees C, pO(2) = 10(-4) to 1 atm, and Delta E = -100 to 100 mV. The DC bias Delta E is used to control the effective pO(2) or oxygen activity at the PCO/ YSZ interface. The non-stoichiometry (delta) of the PCO films is calculated from the measured chemical capacitance (C-chem). These delta values, when plotted isothermally as a function of effective pO(2), established, either by the surrounding gas composition alone, or in combination with applied bias, agree well with each other and to predictions based on a previously determined defect model. These results confirm the suitability of using bias to precisely control delta of thin films in an in situ fashion and simultaneously monitor these changes by measurement of C-chem. Of further interest is the ability to reach effective pO(2)s as high as 280 atm.