Three hydrocarbon phases can coexist at equilibrium at relatively low temperatures in many carbon dioxide (CO2) floods. The formation of an aqueous phase in contact with hydrocarbon phases is inevitable in almost all recovery processes, because of the permanent presence of water in the reservoirs either as injection fluid or as initial formation water. As the number of phases increases, flash calculations become more difficult and time-consuming. A possible approach to reduce the computational time of the phase-equilibrium calculations is to use reduced methods. This paper presents a general strategy to model the phase behavior of CO2/ hydrocarbon/water systems in which four equilibrium phases occur by use of a reduced-flash approach. The speedup obtained by a reduced-flash algorithm compared with the conventional-flash approach is demonstrated for a different number of components and phases. The results show a significant speedup in the Jacobian-matrix construction and in Newton-Raphson (NR) iterations by use of the reduced method when four phases are present. The computational advantage of the reduced method increases rapidly with the number of phases and components. The developed four-phase reduced-flash algorithm is used to investigate the effect of introducing water on the phase behavior of two west Texas oil/CO2 mixtures. The results show changes in the phase splits and saturation pressures by adding water to these CO2/hydrocarbon systems.