Herein, we characterized the micelle structure and hydration behavior of oleic acid-based gemini surfactants by small-angle neutron scattering. A larger counterion size (Li+ < Na+ < K+ < Cs+) resulted in decreased aggregation number (N-agg) of micelles and increased number of water molecules incorporated within the micelle (N-water). We suggested that the incorporated water molecules prevent the formation of lower positive-curvature aggregates such as hexagonal liquid crystals.