High spin transition metal ions (TMIs) important in: a) oxygen transport proteins; or b) photochemical water oxidation catalysis (“water splitting”) will be characterized by novel variable very high frequency EPR methods. TMI complexes frequently exhibit paramagnetic metal cores with multiple unpaired electrons in high spin states. Due to instrumental constraints, investigations of high electron spin states exhibiting large zero field splittings (ZFS) have been limited until now. To lift this restriction, dedicated very high frequency (263 GHz) EPR and Frequency Domain Fourier Transform THz-EPR (FD-FT THz-EPR) methods with increased detection sensitivity and application range will be established. Instrumental upgrades of a synchrotron based FD-FT THz-EPR detection system and the development of novel FD-FT THz-EPR and 263 GHz EPR probe heads will allow single crystal EPR measurements on protein samples and in-situ characterization of high spin ions during water oxidation catalysis, which can involve application of electrical potential and light. Numerical EPR routines based on the public domain EPR simulation package EasySpin will be developed to simulate FD-EPR spectra. The intention is to apply the unique facilities at HZB, in particular FD-FT THz-EPR and 263 GHz EPR to provide information on high spin Mn complexes, currently being developed for photocatalytic “water splitting” as well as high spin Fe in respiratory proteins.