The effects of the degree of neutralization of the acid groups and, to some extent, the precursor melt index on the thermal, rheological, and mechanical properties of two series of poly(ethylene-co-methacrylic acid) (EMAA) ionomers, one based on sodium (Na+) cations and one based on potassium cations (K+), were examined. Differential scanning calorimetry (DSC) and modulus results indicate that the secondary crystallization of the ionomers is generally completed 21 days after melt processing. DSC results indicate that the extent of crystallization increases with increasing neutralization level. The mechanical relaxation seen by dynamic mechanical analysis (DMA) in the vicinity of the secondary crystal melting point shifts to higher temperatures as the neutralization level increases. The theological properties increase with decreasing precursor melt index and with increasing neutralization level to a lesser extent. The ionomer modulus and yield strength increase with increasing neutralization level up to 40% neutralization and then plateau or slightly decrease with further neutralization. The plateaus/maxima may be the result of an optimal spacing of alkaline ions and carboxyl groups within ionic groups at neutralization levels near 33%. The elongation at break and the Izod impact strength decrease with increasing neutralization. The modulus, yield stress, and impact strength are generally lowest for the ionomers with the highest precursor melt index. (C) 2011 Elsevier Ltd. All rights reserved.