Abstract We report on the preparation and spectroscopic properties of complexes of the general formula MoBr 2O 2L n with L=NCCH 3 ( n=2), NCC(CH 3) 3 ( n=1), NCC 6H 5 ( n=2). The complex MoBr 2O 2(CH 3CN) 2 is additionally examined by single crystal X-ray structure determination. The MoBr 2O 2L n complexes are very electron deficient ( 17O-, 95Mo-NMR evidence) and readily react with electron donor N-ligands to give more stable complexes e.g. MoBr 2O 2(bipyrimidine). The structure of the latter compound has been determined by single crystal X-ray crystallography. Reactions with organometallic complexes of the type [Cp′Mo(CN) 2(CO) 2]K (Cp′=Cp or Cp*) and Cp(CO) 2Fe(CN) or [Cp(CO)Fe(CN) 2]K result in the immediate reduction of the Mo(VI) center, as shown by EPR spectroscopy. The MoBr 2O 2L n complexes are catalysts for the epoxidation of cyclooctene but decompose slowly during the course of the reaction due to their pronounced moisture sensitivity.