Oiling out is an unwanted effect during the cooling crystallization of complex molecules from solution: upon cooling, crystallization is preceded by the formation of a second liquid phase (oiling out) which disturbs the crystallization process and often deteriorates the product properties. On the basis of the crystallization of 4,4' dihydroxydiphenylsulfone (DHDPS) from water/acetone mixtures and water/2-propanol mixtures, respectively, this study proved the dependence of the oiling-out behavior on the solvent system. Attempts were made to model the oiling-out phenomenon using a thermodynamic equation of state here: (Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT)). Applying this model, an oiling-out behavior of the ternary system DHDPS/water/acetone could be predicted qualitatively. Exchanging the water/acetone solvent mixture by a mixture of water and 2-propanol led to a prevention of oiling out. This could be predicted by the calculations using PC-SAFT as well as confirmed experimentally. Furthermore, micrographs of the crystals obtained revealed the influence of the oiling out on the properties of the solid product. Size, shape and degree of agglomeration of the particles vary depending on whether an oiling out occurred during the cooling process or not. (C) 2008 Elsevier B.V. All rights reserved.