The cytoplasmic delivery of drugs encapsulated into pH-sensitive liposomes is under the control of a lamellar-to-hexagonal transition. In a previous study, under anhydrous conditions, oligonucleotides (ODN) encapsulated in pH-sensitive liposomes composed of dioleoylphosphatidylethanolamine (DOPE)/oleic acid (OA)/cholesterol (CHOL) were shown to modify the phase behaviour of DOPE. In the present study, the lipid/ODN interactions were evaluated in fully hydrated samples by surface tension measurements, differential scanning calorimetry, X-ray diffraction and turbidimetry. Concerning the lipids, it was shown that OA provoked a disorganisation of DOPE lamellar phases and led to the complete disappearance of hexagonal transition along with heating. The addition of CHOL further decreased the lipid packing in the bilayers. Concerning ODN, these molecules provoked an increase in the surface pressure of a DOPE/OA/CHOL monolayer, indicating the existence of molecular interactions with the lipids. At a supramolecular level, ODN induced a more ordered organisation of DOPE molecules in the lamellar and hexagonal phases, and completely abolished the disorganisational effect of OA and CHOL.