Synthesis of silica aerogels can be markedly accelerated by adding carbon dioxide (CO2) during the sol-gel process. The optimization of the sol-gel process in the presence of CO2 requires the knowledge of the phase equilibrium of the reactants [tetraethyl orthosilicate (TEOS) and water (H2O)], the solvent, and CO2. The perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state is applied here to investigate the conditions under which an undesirable liquid demixing of that quaternary reactive mixture occurs. In this work pure-component parameters of the PC-SAFT equation of state for the solvent acetonitrile (ACN) and TEOS have been identified. Binary interaction parameters of all six related binary systems were determined. The phase behavior of three systems, which are not available in the literature, were studied experimentally: the vapor-liquid equilibria of the systems TEOS-CO2 and TEOS-ACN and the liquid-liquid equilibrium of the system ACN-H2O-TEOS. All four related ternary systems have been calculated at 293.15 K and 20 and 100 bar. The phase behavior of the quaternary system TEOS-H2O-ACN-CO2 is discussed. It is concluded that the addition of CO2 does not lead to a liquid demixing as long as the starting mixture of TEOS-H2O-ACN is homogeneous.