Electron spin resonance (ESR) spectra of Mo5+ in perfluorinated membranes (Nafion) neutralized by MoCl5 suggest that the ionomer acts as an ion-selective medium and replaces the chlorine ligands of the molybdenum center by oxygen ligands from the sulfonic groups. This conclusion is based on an analysis of the ESR parameters that characterize the paramagnetic center and is supported by the detection of only one major paramagnetic molybdenum site, Mo(A) in the bulk ionomer, in the absence of solvents. Ligand design via the ionomer was thus achieved. We propose that the main molybdenum species is MoO(SO3-)(5), where the unique oxygen ligand is from an -S=O group of the sulfonic moiety. Exposure of Mo/Nafion to water (up to 5 Torr) has no significant effect on the ESR parameters and on the intensity of the ESR signal. Exposure to 25 Torr of water leads to the reversible disappearance of the ESR signal. Brief exposure of Mo/Nafion to gaseous acetonitrile (as CD3CN) results in the appearance of two new Mo species, Mo(B) and Mo(C), which are thought to be formed via replacement of one equatorial or one axial -S=O ligand, respectively, based on an analysis of the effect on the corresponding g parallel to values. Soaking of Mo/Nafion samples with acetonitrile results in the reversible disappearance of the ESR signals. All transformations of the ESR signals are explained by the replacement of one L-type ligand (donating two electrons), -S=O in the case of Nafion, by oxygen from water or by nitrogen from acetonitrile. The loss of the signal in the presence of adsorbates is explained by a replacement of one or more of the -S-O- ligands by ligands from the adsorbates. This process is a reversible reductive replacement of the ionic ligands and leads to the concomittant loss of the paramagnetism. The control of ligands around the paramagnetic center, the involvement of the molybdenum in redox processes, and the accessibility of the center to adsorbates are promising features for catalysis. Initial results for the oxidation of ethanol on Nafion and on Mo/Nafion supports are presented. Correlation plots (A(parallel to) vs g(parallel to)) in paramagnetic molybdenum complexes, constructed from data in the literature and measured in this study, show trends that can be used for identification of the number and types of ligands in paramagnetic molybdenum centers.