The polar lipid monoolein (MO) and poly(ethylene glycol), PEG, of different molar mass (1500, 4000 and 8000) were melted, mixed and left to solidify at room temperature. Analysis of the solid mixtures by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) revealed that a phase separation occurs when MO is present in sufficient amounts. The molecular weight of the polymer determines the amount of MO that has to be added before a separate MO phase can be detected. To further understand this behaviour, the folding of the polymers and the thickness of the amorphous domains within the lamellar structure of PEG were determined by calculation of the one-dimensional correlation function from the experimental SAXS data. It revealed that the presence of MO makes the crystalline domains of PEG 1500, which crystallizes unfolded, increase at the expense of the amorphous domains. PEG 4000 and PEG 8000 obtain a higher degree of folding when the MO content in the mixtures increases. Furthermore, a second form of MO was detected when it phase separated from PEG 1500 and 4000. This behaviour was argued to be due to the secondary crystallization of the PEGs. (c) 2005 Elsevier Ltd. All rights reserved.