Lipid assemblies, such as liposomes, comprising transfection enhancer elements (TEE's), which are complexed with the lipid assemblies by means of ionic interactions, or lipids incorporating such TEE's are disclosed for enhancing the fusogenicity of the lipid assemblies. The TEE's have the formula:hydrophobic moiety-pH sensitive hydrophilic moiety (II)The pH sensitive hydrophilic moiety of each TEE is a weak acid having a pka of between 2 and 6 or a zwitterionic structure comprising a combination of acidic groups with weak bases having a pKa of between 3 and 8. Lipids incorporating one or more such TEEs have the formula (I):Lipid moiety-[Hydrophobic moiety-pH sensitive hydrophilic moiety] (I).
