The interaction of DNA with vesicles of cationic lipids mixed with varying amounts of a zwitterionic lipid in dilute solutions was studied by isothermal titration microcalorimetry, differential scanning calorimetry (DSC), and turbidity measurements. Extruded pure dimethyldioctadecylammonium bromide (DODAB) and chloride (DODAC) vesicles and mixed vesicles with various amounts of dioleoylphosphatidylethanolamine (DOPE) were investigated at 25 degrees C. The interaction of DNA with cationic vesicles is fast and endothermic. The enthalpy of reaction per mole of lipid added is constant with varying charge ratio between cationic lipid and DNA but decreases with increasing amounts of the zwitterionic lipid in the vesicle. The enthalpy change is constant up to a critical charge ratio rho+/- of about 0.8 for the addition of lipid vesicles to DNA and 1.2 for the addition of DNA to lipid vesicles. The enthalpy of reaction decreases with the replacement of Br- by Cl- ion and is independent of cationic vesicle size in the range 50-200 nm. The effect of DOPE on the thermotropic behavior of DODAB in aqueous dispersions with and without DNA was investigated using DSC. Inclusion of DOPE in the DODAB cationic vesicle increases the bilayer fluidity, e.g., decreases the melting temperature, T-m. The status of the hydrocarbon chains, solid-like or fluid, has a pronounced effect on the measured enthalpy changes and most strongly for the formation of positively charged, lipid-enriched aggregates.