Aims: During wine making, oxygen and carbon dioxide are often simultaneously present in the liquid phase. We propose a simple rational approach, based on usual chemical engineering and thermodynamic principles, to provide understanding and practical rules for controlling the effects of these two dissolved gases, and especially their inter-relationship. Furthermore, this study proposes an explanation for the "protective" effect against oxidation, which is reported when high concentrations of carbon dioxide are present in musts and wines. Methods and results: The theoretical quantitative relation, termed "binary gas equilibrium line", between the maximum possible concentration of dissolved oxygen in respect to dissolved carbon dioxide was derived and, in our experiments, corresponded to C(O2max) congruent to -0,005 C(CO2) + 7,9 mg.L(-1). Specific saturation experiments using simultaneous injection of air and gaseous carbon dioxide were performed and the experimental results allowed us to validate this theory in the case of gas bubbling in a liquid. Conclusion: It is shown that complete protection is only obtained when carbon dioxide is generated by the fermentation in the liquid. An interesting parallel conclusion is that micro-oxygenation is totally inefficient in such periods. In the case where there is no production of CO(2) but where a high initial dissolved carbon dioxide concentration is present, the "protective" effect acts only by reducing the rate of oxygen transfer. Significance and impact of the study The physical understanding of this phenomenon can be found in the fact that as soon as a gaseous air or pure oxygen phase is in contact with a carbon dioxide saturated liquid, the dissolved carbon dioxide, which is not at equilibrium with the gaseous phase, tends to escape into this gaseous phase. This study points out the complexity of the gas-liquid equilibrium when two dissolved gases are simultaneously present in a liquid and its implication in the winemaking process.