Ion content and molecular weight have been shown to affect the Na-23 NMR spectra of monodisperse sodium-neutralized sulfonated polystyrene ionomers (MNaSPS). A new NMR peak at -2.7 ppm appears at ionization levels above 1.2% and molecular weights of a least 35 000; this peak is not present in polydisperse NaSPS. The fraction of NMR intensity due to this peak is relatively constant above M(n) similar to 100 000. It is proposed that this peak is due to a distorted version of an isolated site. Along with the new -2.7 ppm peak, the monodisperse materials also have far fewer isolated ions than the corresponding polydisperse ionomers. It is possible that the greater chain uniformity of the monodisperse ionomers eliminates plasticization of the aggregates by the low molecular weight components and steric hindrances from the high molecular weight components and permits more complete aggregation of the ionic groups. The new NMR peak can be removed by casting from a cosolvent of THF/water, but not by casting from THF/methanol. Blending of two or more monodisperse materials in solution followed by annealing resulted in an NMR spectrum similar to that of a polydisperse material; however, the behavior of the blend without annealing did not replicate that of polydisperse NaSPS.