The thermodynamic capacity of a species (Ci)in a homogeneous phase is defined as (∂ni/∂µiP, T, njwhereniis the total number of moles ofi per unit quantity of the system irrespective of the actual system chemistry andµiis its chemical potential. Based on this definition, the thermodynamic capacity of oxygen in non-reactive and reactive gas mixtures and in binary and ternary liquid solutions has been computed. For reactive gas mixtures containing stable chemical species which do not undergo significant dissociation such as CO + CO2, H2 + H2O and H2 + CO2, the capacity curves show a maximum at equimolar ratio and a minimum at higher oxygen potentials. If one of the chemical species partly dissociates as in the case of H2S in H2 + H2S mixtures or SO3 in SO2 + SO3 mixtures, capacity curves do not exhibit such maxima and minima, especially at high temperatures. It would be difficult to produce stable oxygen fugacities when the capacity has a low value, for example at compositions near the minimum. Oxygen capacities of non-ideal liquid solutions, Cu-O and Cu-O-Sn, and heterogeneous systems formed at saturation with the respective oxides are discussed.