The local environment of unneutralized carboxylic acid groups in poly(ethylene-ran-methacrylic acid) (E/MAA) ionomers neutralized with monovalent (Li and Na) and divalent (Ca and Zn) ions has been investigated with Fourier transform infrared spectroscopy. These unneutralized acid groups interact with one another to form acid dimers, and they associate with existing neutralized complexes. At room temperature, no free acids can be detected for any system, not even for pure E/MAA. With the acid dimer peak (1700 cm(-1)) and a known unneutralized acid concentration, the concentration of acids associated with a neutralized complex can be determined. This concentration of associated acids increases with increasing neutralization, reaches a maximum below 50% neutralization, and then decreases toward zero near 80% neutralization. This behavior is perhaps due to the increased driving force for aggregation of the neutralization acids. Although Li, Na, and Ca contain similar concentrations of associated acids over the range of neutralizations, the Zn system contains far fewer associated acids (i.e., more acid dimers) at any particular neutralization level. These results are confirmed by an analysis of the absorbance in the neutralized region (1650-1500 cm(-1)).