While the properties of functional oxide thin films often depend strongly on their oxygen stoichiometry, there have been few ways to extract this information reliably and in situ. In this work, the derivation of the oxygen non-stoichiometry of dense Pr0.1Ce0.9O2 thin films from an analysis of chemical capacitance obtained by impedance spectroscopy is described. Measurements are performed on electrochemical cells of the form Pr0.1Ce0.9O2/Y0.16Zr0.84O1.92/Pr0.1Ce0.9O2 over the temperature range of 450 to 800 degrees C and oxygen partial pressure range of 105 to 1 atm O2. With the aid of a defect equilibria model, approximations relate chemical capacitance directly to non-stoichiometry, without need for fitting parameters. The calculated non-stoichiometry allows extraction of the thermodynamic constants defining defect generation. General agreement of these constants with bulk values derived by thermogravimetric analysis is found, thereby confirming the suitability of this technique for measuring oxygen non-stoichiometry of thin oxide films. Potential sources of error observed in earlier chemical capacitance studies on perovskite structured oxide films are also discussed.