The widely accepted 'counterions release' mechanism predicts the existence of charge-neutral cationic lipid/DNA complexes (lipoplexes). Formation of overcharged complexes, both positively and negatively charged, is supposed to be driven by thermodynamics. Here we report a synchrotron small angle X-ray diffraction and dynamic light scattering study aimed to shed light on the mechanism of formation of lipoplexes. By using a two-step process, consisting in adding excess material to preformed isoelectric lipoplexes, we showed that excess DNA does enter preformed complexes while excess lipid does not. Our findings imply that DNA may play a special role in the formation of overcharged lipoplexes. (c) 2006 Elsevier B.V. All rights reserved.