Mixtures of asphaltene + polystyrene + toluene were recently shown to separate into asphaltene-rich and polystyrene-rich liquid phases over a broad range of global compositions at 293 K and atmospheric pressure, for polystyrene mean molar masses of M-w = 393,400 and M-w = 700,000 g/mol. In this work, a model based on the lever rule is used to estimate the binodal curves and the critical points. Measured speed of sound values were compared with values calculated from estimated compositions and speed of sound mixing rules. Large deviations were obtained but the introduction of a parameter gamma, interpreted as the fraction of asphaltenes causing phase separation, improved the agreement between experimental and calculated speeds of sound in the coexisting phases. The best agreement was obtained for gamma = 0.63 and 0.49 for polystyrene molar masses M-w = 393,400 and M-w = 700,000 g/mol, respectively. A model for the depletion flocculation mechanism for mixtures of solvent (toluene), monodispersed colloidal particles and a non-adsorbing polymer (polystyrene) was used to calculate critical loci for cases of theta and good solvents. The mean radii of asphaltene particles active in the phase separation was estimated to fall in the 6-11 nm range. Exogenous SAXS measurements for related asphaltene + aromatic solvent binary mixtures place 1/3-2/3 of asphaltene particles in this nominal size range. The need for coordinated phase equilibrium and asphaltene particle size measurements is underscored. (C) 2012 Elseviei B.V. All rights reserved,