The distribution of the number of oxyethylene groups in molecules of polyoxyethylene (5) nonylphenyl ether (NP5) is shown to significantly affect the structure of NP5/cyclohexane/water reverse microemulsions. The effect is caused by the difference in solubility of NP5 components. This difference leads to fractionation: surfactant molecules dissolved in the intermicellar nonpolar medium have, on average, shorter polyoxyethylene chains than surfactant molecules forming reverse micelles. A model is proposed which takes into account the influence of this fractionation on reverse micelle size and free surfactant concentration. The validity of this model for NP5/cyclohexane/water reverse microemulsions is experimentally confirmed by dynamic light scattering. It is found that changing the degree of surfactant fractionation in the above mentioned microemulsions can result in the formation of non-spherical reverse micelles. (C) 2012 Elsevier Inc. All rights reserved.