The aggregational behavior of a perfluoropolyether ammonium carboxylate in water solution was studied by using the corresponding Cu(II) salt as a spin probe. This allowed us to avoid any possible perturbing effect due to the use of a different hydrophobic chain. The results were found to be consistent with those obtained by different ESR probes or with different techniques (small-angle scattering, IR, etc.), showing the existence of anisotropic aggregates as prelamellar units. At the highest surfactant concentration, where lyotropic phases are known to be present, a remarkable counterion binding ability of the COO- interfacial groups was evidenced. Details on the state of water in the interlamellar region were also provided.