Small angle neutron scattering has been used to study the aggregation behaviour of a sodium sulfonated polystyrene ionomer (SPS) with a molecular weight of 10(5) g mol(-1) and a sulfonation level of 0.95 mol% in dilute p-xylene solution. The variation with concentration of both the average aggregate molecular weight and the average aggregate radius of gyration at temperatures ranging from 25 to 60 degrees C were obtained. All the results observed can be quantitatively interpreted by use of a closed association model in which single chains are assumed to be in equilibrium with aggregates of one size. At concentrations below 0.1 g dl(-1) this ionomer exists in solution mainly as single collapsed chains with an average radius of gyration of 78 Angstrom. By comparison, the single chain dimension of the unsulfonated polystyrene at infinite dilution is 130 Angstrom in xylene. As the ionomer concentration is raised, an increasing fraction of the single chains associates to form small compact aggregates with a radius of gyration of 150 Angstrom and consisting of three chains. Both the single chain and aggregate dimensions are temperature independent, as is the extent of aggregation at a given concentration. This shows that the free energy change on forming aggregates from single collapsed chains is primarily entropic in nature. (i.e. there is no enthalpy change on aggregation). Moreover, no enthalpic contribution to the foe energy of mixing for the ionomer aggregates with the solvent is observed. A contrast matching method was used to probe the aggregate internal structure. Within an aggregate the individual chains were found to expand from their collapsed state to 115 Angstrom. The combined results suggest that the aggregates and single chains consist of a compact core which is primarily ionic in nature surrounded by solvent swollen polystyrene chains. In this picture the aggregates would form because of the increase in both configurational entropy of the polymer chains and the greater mixing of the polystyrene chains with the solvent on single chain expansion within the aggregates.