Small-angle neutron scattering has been used to determine the dimensions of single chains within the aggregates formed in the solvent p-xylene by a sodium sulfonated polystyrene ionomer (SPS). This polymer has a molecular weight of 10(5) g mol(-1) and sulfonation level of 1.25 mol %. The dimensions were obtained by using a mixture of normal and deuterated polymer to achieve contrast match of the overall polymer in a mixed normal and deuterated solvent. As the concentration of the ionomer was raised, the average single-chain radius of gyration increased. This rise occurs as a result of an increase in the ratio of aggregates to single chains. The isolated ionomer chains collapse to dimensions smaller than for the ’parent’ polystyrene in a theta solvent. Within an aggregate, however, each individual ionomer chain expands to a size larger than that for the equivalent polystyrene in p-xylene (a good solvent for the ’parent’ polymer). Using an open association model to quantify the extent of aggregation as a function of concentration, the single-chain dimension both in the aggregates and as collapsed chains have been estimated. The average radius of gyration of a single chain was found to be 60 Angstrom for an isolated chain but rose to a maximum value of 144 Angstrom within an aggregate. It is suggested that the entropy change that occurs on aggregation arises primarily from the single-chain expansion. The models developed have also been used to explain previous measurements of the single-chain dimensions of other ionomers in non-polar solvents.