Interactions of a-amino acids, glycine (gly), valine (val), leucine (leu) and phenylalanine (phe) and the closely related phenylalaninamide (phea) with cationic vesicles formed by dimethyldioctadecylammonium bromide (DOAB), dihexadecyldimethylammonium bromide (DHAB), with anionic vesicles formed by sodium ditetradecyl phosphate (DTP) and with zwitterionic vesicles formed by dimyristoyl (C14:0) phosphatidylcholine (DMPC) have been probed using differential scanning microcalorimetry (DSC). Gel-to-liquid transitions for DTP, DOAB, DHAB and DMPC vesicular systems are relatively unaffected by added alpha -amino acids. In contrast added phea markedly changes the recorded scans for DSC for DTP and DMPC vesicular systems indicating that small changes in the structure of added solutes can have dramatic effects on the structures and thermal characteristics of the vesicles in aqueous suspensions. The results indicate, perhaps surprisingly, that alpha -amino acids are not incorporated into the bilayers whereas phea is incorporated into DTP and DMPC bilayers.