Thin film non-stoichiometric oxides are of interest as electrodes and electrolytes in electrochemical energy conversion devices including solid oxide fuel cells and electrolyzers because of their potential to lower device operating temperatures and as model systems to aid in investigating rate limiting kinetics. Many of these non-stoichiometric oxides undergo chemical expansion, or composition-dependent volume change, upon ‘breathing’ of oxygen that can result from variations in temperature, oxygen partial pressure, and electrical bias. However, thin film responses are not always accurately predicted based on bulk models or experiments. Understanding the time-resolved operando chemical expansion of non-stoichiometric oxide thin films is essential for modeling and predicting the development of stress and strain in actual devices during temperature excursions or gas interruption. In this work, we report a novel method for measuring chemical expansion of thin films of non-stoichiometric oxides at temperatures up to 650°C and demonstrate its application to the model fluorite-structured system (Pr, Ce)O_2-δ (PCO). In this technique, we vary the oxygen activity of the material of interest by applying an oscillating electrical bias across a supporting yttria stabilized zirconia (YSZ) electrolyte, and use a high-temperature, depth-sensing instrumented indenter to detect the amplitude and phase lag of the material’s expansion response, allowing us to characterize nm-scale chemical expansion in PCO films with sub-second-scale resolution.  We relate the observed frequency and temperature-dependent response to fundamental processes governing expansion kinetics and compare our results with available defect models for PCO. This ‘electrochemomechanical spectroscopy’ provides an accessible means of characterizing how factors such as film thickness, composition, or microstructure, influence the operando chemomechanical deformation of non-stoichiometric oxide-based devices on short time scales.