The critical properties of binary mixture (cyclohexane + carbon dioxide) involved in the selective oxidation of cyclohexane by oxygen in the presence of carbon dioxide were measured using a high-pressure viewing cell with visual observation. Moreover, the critical properties of the nominal reacting mixture (cyclohexane + nitrogen + carbon dioxide + cyclohexanol + cyclohexanone + water, where nitrogen is regarded as the homomorphous stand-in for oxygen) were also determined with the composition representing the extent of reaction (conversion of cyclohexane) with the initial mole ratio of cyclohexane to oxygen being 1:1 and 2:1 and the mole fraction of carbon dioxide being 0.5, 0.6, and 0.7, respectively. The results indicated that the critical properties of the reacting mixture depend on the extent of reaction as well as the initial composition, and the addition of proper solvent is an effective way to tune the critical properties of the reacting mixture. Such information is essential for determining the reaction conditions when one intends to carry out the selective oxidation of cyclohexane by oxygen under supercritical conditions and/or using carbon dioxide as a solvent, which could be an interesting option to enhance the reaction rate, improve the selectivity, and prolong the catalyst lifetime compared with current processes in the gas or liquid phase.