Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag2O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO1-x was formed in O-2+O-3 gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag2O, Ag2O to AgO1-x and AgO to Ag2O3 were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 x 10(7) Pa. In the O-2+O-3 mixture at ambient pressure containing 5 mole percent O-3, the oxygen potential is similar to 112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is similar to 4 x 10(17) Pa. Thus, metastable species such as O-3 or charged species such as O- present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.