Complexes of the type (η5-C5R5)Mo(CO)3X (X = Me, Cl; R = H, Me), being efficient homogeneous catalysts for the epoxidation of olefins, have been examined for their catalytic performance at 55 °C in systems containing room temperature ionic liquids (RTILs) of composition [BMIM]NTf2, [BMIM]PF6, [C8MIM]PF6 and [BMIM]BF4. The catalytic performance for cyclooctene epoxidation depends strongly on the water content of the system, the catalyst solubility in the RTIL, and the reaction behaviour of the RTIL under the applied reaction conditions. The catalysts can be recycled without significant loss of activity when a reaction system containing [BMIM]NTf2 and [BMIM]PF6 in a 4:1 relationship is used. High proportions of [BMIM]PF6 lead to a ring opening reaction (diol formation), due to HF formation and the presence of residual water.Graphical abstract