Liposomes, or vesicles, have been studied extensively both as models of biological membranes and as drug delivery vehicles. Typically it is assumed that all liposomes within the same preparation are identical. Here by employing pairs of fluorescently labeled lipids we demonstrated an up to 10-fold variation in the relative lipid composition of individual liposomes with diameters between 50 nm and 15 mu m. Since the physicochemical properties of liposomes are directly linked to their composition, a direct consequence of compositional inhomogeneities is a polydispersity in the properties of the individual liposomes in an ensemble.