A small-angle neutron scattering (SANS) study of aqueous solutions of polyethylene-co-methacrylic acid partially neutralized ionomers is presented. A series of polymers presenting different degrees of polymerization, degrees of neutralization, counterions, and molecular weights are studied. The modeling of the dilute solution spectra combined with the analysis of the large-angle asymptotic behavior of the concentrated solutions spectra allowed us to conclude that these polymers form ver. large colloidal particles which are well described as either finite length cylinders or prolate ellipsoids. It is shown that the most important parameter defining the size of the particles is the ionic content. The radius varies from 55 to 135 Angstrom and is proportional to the average distance between ionized carboxylated groups while the length of the particles increases dramatically for low ionic content. A transition from almost spherical particles to cylinders was evidenced. The effect of the concentration and of the molecular weight can be considered as negligible while the effect of the counterion is a modification of the eccentricity of the particles. The SANS spectra of concentrated solutions present a well-defined maximum. The position of the scattering maximum can be calculated using space-filling arguments, and the shape of the interference term is well reproduced by a Hayter and Penfold analysis.