A model for the relationship between W/O-aggregate geometries and molecular species constituting the aggregates was proposed. The microemulsion phase in equilibrium with an excess aqueous phase was discussed. We restricted the surfactants to ionic ones in which head groups electrostatically interact with each other. The hydrophobic groups were modelled by the density distribution of the chain segments within the surfactant tails in the hydrophobic layer coating the aggregate. The interaction between the hydrophobic groups of surfactants was incorporated based on the excluded volume interaction of the segments. On this basis, we analytically expressed the chemical potential of the surfactants in terms of microscopic parameters characterizing the nature of the surfactants. Following the thermodynamical equilibrium condition, the surfactant systems were classified into four types : systems forming spherical aggregates, systems forming cylindrical aggregates, to form aqueous micelles or O/W aggregates and to form no small aggregates. The domain for the four types were drawn on the maps, of which abscissa and ordinate represent the strength of the electrostatic interaction and the length-ratio of the hydrophilic/hydrophobic layers, respectively. The packing of the segments within the hydrophobic tails played a key role to determine the aggregates geometry. The predictions by the present model were qualitatively consistent with some experimental observations.