New liquid-liquid equilibrium (LLE) data are presented for the quaternary system of water + methanol + heptane + methylbenzene at T = 298.15 K under atmospheric conditions. The quaternary mixtures were prepared by mixing pure water, methanol, and an equimolar mixed (heptane + methylbenzene). A quinary system containing these compounds and dimethyl carbonate (DMC) was also studied at the same temperature. The quinary mixtures were obtained with three mole fractions (0.25, 0.50, and 0.75) of DMC in the binary (methanol + DMC) mixtures. The compositions of liquid phases at equilibrium were determined by gas-liquid chromatography. The consistency of the experimental LLE data has been confirmed according to the Othmer-Tobias and the Hand correlations. The water composition in the organic phase and the hydrocarbon solubility in the aqueous phase were analyzed in terms of (methanol + DMC) in the global composition. The distributions of methanol and DMC between the aqueous phase and the organic phase were also discussed. The experimental LLE data were correlated with the nonrandom two-liquid (NRTL) and the universal quasichemical activity coefficient (UNIQUAC) solution models, and the binary interaction parameters were collected. The density and refractive index at each LLE composition of each conjugate phase were measured at T = 298.15 K. Densities were determined using a vibrating-tube densimeter. Refractive indexes were measured using a digital Abbe-type refractometer. The solubility data were obtained by the cloud point method for the pseudoternary system with water (1) + (methanol + DMC) (2) + (0.50 heptane + 0.50 methylbenzene) (3) at T = 298.15 K. The system exhibited a large partially miscible region in the phase diagram of Gibbs triangle. The region of heterogeneity was found to decrease with an adequate blending of DMC with methanol.