Even though RNA interference is a highly specific technique to selectively silence the expression of any gene, the delivery of RNAi molecules remains a challenge. Recently, we developed a clinically acceptable efficient formulation of siRNA. This delivery system consisted of the cationic lipid 2-{3-[Bis-(3-amino-propyl)-amino]-propylamino}-N-ditetradecyl carbamoyl methyl-acetamide, or DMAPAP, (ii) the neutral lipid 1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine or DOPE and (iii) an anionic polymer that enhances lipoplexes efficiency. We show here that this enhancement of efficiency is due to higher stability of polymer containing-siRNA lipoplexes, leading to longer retention of siRNA integrity. We assayed siRNA integrity upon incubation of siRNA lipoplexes in various biological media using electrophoresis and Fluorescence Correlation Spectroscopy. We show that the addition of anionic polymer provided enhanced stability of incorporated siRNA mainly in saline buffer at room temperature and in human serum at 37 °C and did not interfere with the release of siRNA from lipoplexes in modeled cellular media.