A Thesis submitted to the Universities of Leipzig and Turin in candidature for a Joint PhD degree by Paolo Cleto Bruzzese Abstract Atomically dispersed transition metal ions in zeolites catalyse a wide range of industrial reactions and are at the centre of intense research interest to design new sustainable synthetic pathways for energy conversion and environment remediation. One of the big challenges in this context is the characterization and location of the active sites. Indeed, mapping their nature with atomic-scale precision occupies a central place in the theory and practice of heterogeneous catalysis. In this thesis, the site-selectivity and sensitivity of Electron Paramagnetic Resonance (EPR) with its pulsed variants are combined with quantum chemical modelling to determine the microscopic structure of monomeric CuII species in zeolites with Chabazite (CHA) topology as a function of the hydration conditions and sample composition. By isotopic labelling of the zeolite framework with 17O and employing 17O ENDOR spectroscopy, the degree of covalency in the Cu-O bond is mapped and the evolution of CuII sites as a function of the hydration conditions is followed. By combining 1H HYSCORE experiments with state-of-the-art quantum chemical modelling, the EPR signature of the redox active hydroxo-CuII species is univocally identified and a quantitative assessment of its electronic and geometric structureis provided as a function of zeolite composition.