Vesicles were prepared from pure phosphatidylcholines (PC), phosphatidic acids (PA), or dioctadecyldimethylamonium bromide (DODAB). The aggregate structure was examined above and below the gel-to-liquid crystalline transition temperature (T-m) by means of cryo-transmission electron microscopy. The redox reaction between a membrane bound viologen and dithionite was studied in the different lipid systems. It was found that the presence of faceted vesicles or open bilayer fragments, below T-m, lead to double-exponential reduction kinetics.