Mixed matrix membranes (MMMs) containing various amounts of the metal-organic framework (MOF) Cu3BTC2 as filler in P84 were characterized in terms of their ethylene and ethane separating performance. Previous research showed that especially the use of the MOF Cu3BTC2 improves the ethylene/ethane separating ability due its selective interaction with the olefin. Although the ethylene permeability remained constant, the ethylene/ethane permeability selectivity significantly increased to a value of 7.1 with increasing Cu3BTC2 loading. Experiments show that the ethylene solubility coefficient increased from 1.0 to 2.9×10−3 mol/(m3 Pa) with increasing Cu3BTC2 loading up to 20 wt%. Since the ethylene permeability coefficient remained constant at 17×10−18 mol m/(m2 s Pa) with increasing Cu3BTC2 loading, the ethylene diffusion coefficient was calculated to decrease by a factor of three. Evaluation of the reason for the strong increase in permeability selectivity with increasing Cu3BTC2 loading revealed that this is the result of an increase in diffusion selectivity by a factor of two. These results suggest immobilization of ethylene inside the MOF particles as the result of strong ethylene–copper(II) MOF interactions.