A short retrospection is given on the development of Mossbauer spectroscopy and on the early period of its application to heterogeneous catalysis in Hungary. Besides the historical view, we present results of recent studies of a latest application on a nanosystem. Namely, catalytic centres in Fe-ZSM-5 are studied by in situ spectroscopy, the results are correlated with parallel activity measurements in the decomposition of N2O. Changes in paramagnetic relaxation features are interpreted as the effects of Fe2+/Fe3+ interconversions. From the analysis, the dominance of the mononuclear species is suggested at lower temperature (620 K). The dinuclear Fe(3+) (framework)-O-Fe(2+) (extra-framework) species play probably more expressed role at higher temperatures (similar to 770 K) in the decompositon of nitrous oxide.