The use of a novel technique capable of measuring in-situ oxygen mass changes in solid oxides is reported. This technique comprises an inertial, tapered element oscillating microbalance (TEOM), the operation of which is based on the natural frequency of an oscillating tapered element containing the material and its mass. TEOM was employed to determine oxygen stoichiometries as a function of temperature and oxygen partial pressure for a perovskite oxide, namely La0.2Sr0.8Co0.8Fe0.2O3-delta. Experiments were performed at temperatures between 300 and 500 degreesC and oxygen partial pressures between 5 X 10(-4) and 1.3 X 10(-3) atm where gas atmospheres of well-defined oxygen partial pressures were generated by means of a solid state electrochemical cell. It was found that, at the conditions under consideration, oxygen stoichiometry decreases with increasing temperature and decreasing oxygen partial pressure. Oxygen stoichiometries were also independently determined by means of solid electrolyte coulometry (SEC) at temperatures between 300 and 600 degreesC and oxygen partial pressures between 1.3 X 10(-4) and 1.3 X 10(-3) atm. The results obtained by SEC were found to be in good agreement with those obtained by TEOM for the range of common operating conditions.