The dilute solution viscosity was investigated for several polystyrene-based cationic ionomers. It was found that intramolecular aggregation among the ionic groups was strongly dependent on the sizes of quaternary onium groups and counter anions. The extent of the aggregation was controlled by the solvent polarity and the solvation to the ionic groups. When there was a strong selective solvation to small counter ions, the structure of onium groups shows a minor effect on the viscosity behavior, indicating little aggregation among the ionic groups. A strong solvation to small quaternary cations also eliminates the intramolecular aggregation and the influence of counter ions was barely observable. When the selective solvation to counter ions was disabled by the enlarged size of the counter ions, however, the viscosity depended on onium group structure or spacer chain length. If the solvent solvates neither counter ions nor quaternary groups, the smaller the sizes of onium cation and counter anion, the lower the reduced viscosity due to an enhancement of the ionic aggregate formation.