The bonding of Mo-V dispersed in poly(ethylene-co-methacrylic acid) (EMAA) ionomers was deduced from ESR measurements at X (9.5 GHz) and W (95 GHz) frequencies, spectral simulations, and FTIR spectra and compared with previous results for Mo-V in polyacrylic acid (PAA) and in perfluorinated ionomers ("Nafion") that contain sulfonic acid groups. Molybdenum dispersed in EMAA ionomers was obtained by grafting the ionomer with gaseous MoCl5 in the absence of air, to prevent oxidation to Mo-VI. This method produced both physically and chemically bonded Mo-V species, and the ratio of the two species depended on the temperature of MoCl5 during grafting. The type of ligands was deduced from the g-values, and the hyperfine splittings of magnetic Mo nuclei (I = 5/2). The major species detected at the lower grafting temperature was Mo-V bonded to five chlorine ligands and to an oxygen ligand from a carboxylic group of the ionomer. Heating in vacuum to 400-500 K ("activation") led to the gradual replacement of the chlorine ligands by oxygen ligands and to the removal of some molybdenum. The nature of the oxygen ligands was deduced from FTIR spectra and depends on the methacrylic acid (MAA) content in the ionomer : in Mo/EMAA samples activated at approximate to 400 K, molybdenum is bonded to the substrate by bidentate carboxylic groups in the ionomer containing 1.3 mol % MAA and by monodentate carboxylic groups for an MAA content of 11.2 mol %. Exposure of Mo/EMAA samples to ethanol vapor resulted in an increase in the total ESR signal intensity and in the solubilization of ESR-silent Mo-V species; at 300 K the ESR spectra are isotropic, in contrast to the anisotropic signals observed at the same temperature in the absence of solvents. For water as adsorbate, two Mo-V species differing in line widths were detected and assigned to Mo-V species located in the water pools of the ionomer and in the interior of the nonpolar domains, respectively. This result is in support of the model proposed for EMAA ionomers swollen by water, which was based on ESR spectra of amphiphilic spin probes. The bonding of Mo-V to EMAA ionomers is similar to that described previously for Mo/PAA and different from that in Mo/Nafion, indicating that the determining factor is the type of acid in the ionomers. Small differences between the Mo/EMAA and Mo/PAA systems were assigned to the different distribution of carboxylic groups in the polymer.