AbstractLine shapes of electron paramagnetic resonance spectra of Mn2+‐ions in a sulfonated polystyrene ion‐exchange resin at varying degrees of hydration and temperatures have been examined at two microwave frequencies, 9 and 35 GHz. From a comparison between measured and simulated EPR‐spectra, the correlation times for the process modulating the zerofield splitting (zfs) interaction and the magnitude of the zfs‐parameters have been determined. For all samples with a water concentration higher than three molecules per sulfonate group, the correlation times were in the range of 10–40 ps, indicating a relatively high mobility of the hydrated cations in the polymer matrix. At lower water concentrations, forbidden transitions (Δm = ±1) have been observed. Their intensities increase with falling water concentrations and temperatures. Their temperature dependence was used to determine the fraction of metal ions associated at low water concentrations with the anchored sulfonate groups for a mean life‐ti