Oiling out, an undesirable effect of the crystallization of organic molecules from solution, usually disturbs the crystallization process and deteriorates the product properties. In this work the purification of raw idebenone that oils out was firstly attempted by use of both drowning out and cooling crystallization from hexane/methylene chloride mixtures. Experimental results have shown that, oiling out can greatly affect the outcome of crystallization: at low initial concentrations, oiling out did not take place and crystallization proceeded normally to harvest the product with high purity; while at high initial concentrations, oiling out frequently occurred and crystallization was hindered or even stopped. In addition, such parameters as the polarity of solvent, cooling rate, seeding and addition rate of antisolvent can also be utilized to avoid the occurrence of oiling out so as to increase the purity of crystallization product. (C) 2012 Elsevier B.V. All rights reserved.