Morphology and mechanical properties of nanocomposites prepared by melt mixing a montmorillonite-based organoclay with lithium, sodium, and zinc ionomers of poly(ethylene-co-methacrylic acid) are presented here. The effect of the type of neutralizing cation on the melt rheology and organoclay exfoliation efficiency of the ionomers was examined using transmission electron microscopy, wide-angle X-ray scattering, DSM axial-force rheometry, and stress-strain analysis. Nanocomposites prepared from the sodium and the zinc ionomers displayed much better exfoliation of the organoclay and superior levels of reinforcement compared to equivalent nanocomposites prepared from the lithium ionomer. On the basis of the detailed experimental analysis, the lower levels of organoclay exfoliation observed in the nanocomposites prepared from the lithium ionomer are suggested to be a result of irreversible exchange of quaternary ammonium ions (of the organoclay) for the very small lithium ions (of the polymer) that can enter the montmorillonite lattice structure.