Actuator operation in increasingly extreme and remote conditions requires materials that reliably sense and actuate at elevated temperatures, and over a range of gas environments. Design of such materials will rely on high-temperature, high-resolution approaches for characterizing material actuation in situ. Here, we demonstrate a novel type of high-temperature, low-voltage electromechanical oxide actuator based on the model material PrxCe1-xO2-delta (PCO). Chemical strain and interfacial stress resulted from electrochemically pumping oxygen into or out of PCO films, leading to measurable film volume changes due to chemical expansion. At 650 degrees C, nanometre-scale displacement and strain of >0.1% were achieved with electrical bias values <0.1V, low compared to piezoelectrically driven actuators, with strain amplified fivefold by stress-induced structural deflection. This operando measurement of films 'breathing' at second-scale temporal resolution also enabled detailed identification of the controlling kinetics of this response, and can be extended to other electrochemomechanically coupled oxide films at extreme temperatures.