Photoresponsive catanionic vesicles have been developed as a novel gene delivery vector combining enhanced cellular uptake with phototriggered release of vesicle payload following entry into cells. Vesicles with diameters ranging from 50 to 200 nm [measured using cryo-transmission electron microscopy (TEM) and light-scattering techniques] form spontaneously, following mixing of positively charged azobenzene-containing surfactant and negatively charged alkyl surfactant species. Fluorescent probe measurements showed that the catanionic vesicles at a cation/anion ratio of 7:3 formed at surfactant concentrations as low as 10 mu M of the azobenzene surfactant tinder visible light (with the azobenzene surfactant species principally in the trans configuration), while 50-60 mu M of the azobenzene surfactant is required to form vesicles under UV illumination (with the azobenzene surfactant species principally in the cis configuration). At intermediate surfactant concentrations (ca. 15-45 mu M) under visible light conditions, transport of DNA-vesicle complexes occurred past the cell membrane of murine fibroblast NIH 3T3 cells through endocytosis. Subsequent UV illumination induced rupture of the vesicles and release of uncomplexed DNA into the cell interiors, where it was capable of passing through the nuclear membrane and thereby contributing to enhanced expression. Single-molecule fluorescent images of T4-DNA demonstrated that the formation of vesicles with a net positive charge led to compaction of DNA molecules via complex formation within a few seconds, while UV-induced disruption of the vesicle-DNA complexes led to DNA re-expansion to the elongated-coil state, also within a few seconds, Transfection experiments with eGFP DNA revealed that photoresponsive catanionic vesicles are more effectively taken tip by cells compared to otherwise identical alkyl (i.e,. nonazobenzene-containing and thus nonlight-responsive) catanionic vesicles, presumably because of pi-pi stacking interactions that enhance bilayer rigidity in the photoresponsive vesicles. Subsequent UV illumination following endocytosis leads to further dramatic enhancements in the transfection efficiencies, demonstrating that vector unpacking and release of DNA front the carrier complex can be the limiting step in the overall process of gene delivery.