Small angle neutron scattering has been used to determine the dimensions and extent of aggregation of dilute solutions of sodium sulfonated polystyrene (SPS) ionomers of molecular weight 10(5)gmol(-1) and sulfonation levels of 0.95 and 1.65 mol% in p-xylene. At concentrations below 0.1x10(-2)gml(-1) the 0.95 mol% SPS was present as mainly single chains of considerably smaller dimension than that of the ’parent’ polystyrene. Above this concentration small compact aggregates consisting of three chains were formed. These data could be analysed using the closed association model. The higher sulfonation level ionomer was present as even more compact single chains in very dilute solution. It associated, however, to form a distribution of much less dense aggregates with an average size that increased as the polymer concentration was raised. This type of behaviour was interpreted using the open association model and used to explain why semi-dilute solutions of this latter ionomer can gel.