Oiling out denotes a (metastable) liquid liquid demixing during cooling crystallization prior to formation of the first crystals. This in most cases unwanted effect deteriorates the properties of the desired solid product. On the basis of the crystallization of the model substance polyethylenglycoldimethylether (PEGDME) from pure solvents, the influence of the molecular size of the solute and the type of solvent on the oiling-out behavior was systematically investigated. In this study the solubility data were determined gravimetrically as well as by using differential scanning calorimetry. The crystallization and oiling-out temperatures were detected visually in batch crystallization experiments. Oiling out was observed during the crystallization of PEGDME with a molar mass of 2000 g/mol (PEGDME2000) from diethylketone, ethyl acetate and 2-propanol, whereas no oiling out was detected during the cooling process of PEGDME with a molar mass of 1000 g/mol (PEGDME1000) from all solvents considered. Furthermore the oiling-out temperature for PEGDME2000 was not significantly influenced by the chosen solvents diethylketone, ethyl acetate and 2-propanol. In the second part of this study, it is shown that the appearance and absence of oiling out in all considered solvents can be qualitatively predicted by the pertubed chain statistical association theory (PC-SAFT) only using solubility data. (C) 2010 Elsevier B.V. All rights reserved.