Following the method introduced by Smith and Missen a complete and independent set of chemical equilibria describing the chemical equilibrium in molten carbonate under standard cathode conditions was obtained [1]. With a modification of this method, recently reported by Coleman and White, (electro)chemical equilibria are included in the description [2]. A complete set of chemical equilibria can consist of the dissociation equilibrium and one formation equilibrium in terms of CO32-, CO2 and O-2 for each reduced oxygen species postulated to be present in the melt. These formation equilibria can be obtained from a general expression derived here. A corresponding electrochemical set can be obtained by adding the well known MCFC overall cathode equilibrium to the chemical set. It follows easily from the representation with the base set and using the phase rule, that the number of degrees of freedom of the system does not depend on the number crf reduced oxygen species assumed present in the melt. In practice the free parameters are temperature, the partial pressure of carbon dioxide and oxygen and for mixtures, the melt composition. Finally it is explained that humidification of the cathode gas does not change the (electro-)chemical oxygen system, i.e. the concentrations of the reduced oxygen species are not directly affected by the presence of steam in the cathode gas. A good understanding of these equilibrium conditions is very helpful when performing non-equilibrium studies, as presented in part II of this series, and in studies of the oxygen reduction and corrosion mechanisms in the MCFC.