The effect of counterions on vesicles prepared from dihexadecyl-phosphate is investigated. Alkali metal ions as well as an organic guanidinium counterion have been employed and it has been found that the size of the vesicles is dependent on the size of the counterions, Specifically, the bigger the counterion the smaller are the vesicles that are formed. The vesicles exhibited very good stability which was not significantly affected by the nature of the counterion, It is interesting to note that a subtle chemical modification, viz., in the nature of the counterion, can have a significant bearing on the size and morphology of the vesicles. The implications in self-assembly occurring in biological systems are therefore easy to appreciate.