Magnetization, optical absorbance, and 19F NMR spectra of Nafion transparent films as received and doped with Mn2+, Co2+, Fe2+, and Fe3+ ions with and without treatment in 1H-1,2,4-triazole (trz) have been studied. Doping of Nafion with Fe2+ and Co2+ and their bridging to nitrogen of triazole yields a hybrid self-assembling paramagnetic system that exhibits interesting magnetic and optical properties. These include spin crossover phenomena between high-spin (HS) and low-spin (LS) states in Nafion-Fe2+-trz and Nafion-Co2+-trz accompanied by thermochromic effects in the visible range induced by temperature. A large shift of the magnetization curve induced by a magnetic field in the vicinity of the HS ? LS, similar to 220 K, observed for Nafion-Fe2+-trz has a rate of similar to 6 K/kOe, which is about three orders of magnitude larger than that in bulk spin crossover Fe2+ materials. Selective response of 19F NMR signals on doping with paramagnetic ions demonstrates that NMR can be used as spatially resolved method to study Nafion film with paramagnetic network. Both chemical shift and width of 19F NMR signals show that SO?3- groups of Nafion, Fe or Co ions, and nitrogen of triazole are bonded whereas they form a spin crossover system. Based on a model of nanosize cylinders proposed for Nafion [K. Schmidt-Rohr and Q. Chen, Nat Mater (2008), 75], we suggest that paramagnetic ions are located inside these cylinders, forming self-assembling magnetically and optically active nanoscale networks. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 129138, 2012