Cationic liposomes spontaneously interact with negatively charged plasmid DNA to form a transfection competent complex capable of promoting the expression of a therapeutic gene. This work aims to improve the understanding of the poorly defined mechanisms and structural rearrangements associated with the lipid-DNA interaction. Specifically, dimethyl dioctadecylammonium bromide (DDAB):dioleoyl phosphatidylethanolamine (DOPE) and 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) liposomes were mixed with a reporter plasmid (pADbeta or pCMVbeta) to form lipid-DNA complexes. The size and charge characteristics of the complexes as determined by photon correlation spectroscopy and microelectrophoresis were found to be dependent on the lipid:DNA ratio, with both DDAB:DOPE-DNA and DOTAP-DNA complexes aggregating at around neutral zeta potential. Negative stain transmission electron microscopy demonstrated at least three distinct complex structures being formed at the same DOTAP:DNA ratio. We postulate that two of these aggregates are structural moieties involved in the formation of the efficient transfection particle. Gel electrophoresis was used to determine the efficiency and extent of lipid-DNA complex formation. Results showed that only DOTAP liposomes were capable of preventing ethidium bromide intercalation with DNA and protecting the enclosed plasmid from nuclease digestion. When a range of lipid-DNA complexes were transfected into in vitro cell lines, the efficiency of reporter gene (beta-galactosidase) expression was found to depend on the type of liposome used in the complex, the ratio of lipid:DNA and the transfected cell line. Our results challenge the requirement for DOPE to be included in the formulation of cationic lipid vectors, especially in the case of DOTAP containing liposomes.