Nanocomposites were prepared by melt blending various sodium (Na+) and potassium (K+) ionomers formed from poly(ethylene-co-methacrylic acid) and the M-2(HT)(2) organoclay formed from montmorillonite (MMT). The effects of the neutralization level of the acid groups and the precursor melt index on the morphology and properties of the nanocomposites were evaluated using stress-strain analysis, wide angle X-ray scattering (WAXS), and transmission electron microscopy (TEM) coupled with particle analysis. The aspect ratio generally increases as the neutralization level increases, except for Na+ ionomer nanocomposites with neutralization levels >50%. It appears from both WAXS and TEM analyses that Na+ ionomer nanocomposites have higher levels of MMT exfoliation and particle orientation in the flow direction than K+ ionomer.nanocomposites. DSC results indicate that the level of crystallinity in the Na+ ionomers generally increases slightly with MMT addition, while the crystallinity in the K+ ionomers decreases slightly with MMT addition. The relative modulus of K+ ionomer nanocomposites increases as the degree of neutralization increases. The relative moduli of Na+ ionomer nanocomposites are higher than the relative modulus of K+ ionomer nanocomposites, likely due to the increased crystallinity of the Na+ ionomers and the decreased crystallinity of the K+ ionomers upon addition of MMT, the higher exfoliation levels measured by the aspect ratios and the particle densities, and the higher particle orientation indicated by TEM and WAXS. The relative modulus generally increases as the aspect ratio increases. The elongation at break generally decreases as the MMT content increases and as the neutralization level increases for both ionomer types. The fracture energy of most of the ionomers increases with the addition of MMT, reaches a maximum between 2.5 and 5 wt% MMT, and then decreases upon further MMT addition. (C) 2011 Elsevier Ltd. All rights reserved.