Three novel tridentate pyridazine phenolate ligands were prepared in high yields by Schiff-base condensation of salicylic aldehyde with various pzridazine hydrazines (substituent R in the 6 position: R = Cl (HLCl), Bu-t (HL'Bu), or tol (HLtol)). They react with [ReOCl3(OPPh3)(SMe2)] to form rare mononuclear trans-dichloro oxo complexes of general formula [ReOCl2(L-R)] with R = tol (1), Bu-t (2), or Cl (3) as confirmed by single-crystal X-ray diffraction analyses of 1 and 2. They were found to be catalysts for oxidation of cyclooctene to the corresponding epoxide by tert-butyl hydroperoxide (TBHP). Extensive UV-vis and NMR spectroscopic investigations followed by evaluation using the powerful Mauser method revealed mechanistic details. This showed the catalyst precursor [ReOCl2(L)] (2) to be transformed into the rhenium(VII) compound [ReO3L] (4) in a two-step reaction via intermediate INT which is tentatively assigned to [ReO2L]. Confirmation gave the isolation of 4 by reaction of 2 with excess of TBHP. Monitoring the catalytic oxidation reaction by UV vis spectroscopy clearly excludes the two rhenium(V) compounds 2 and INT from being the catalytically active species as their formation is several orders of magnitude faster than the observed catalytic epoxidation reaction.