The influence of different diluent gases (He, Ar, N-2, CH4, and CH4 balanced with He) on the epoxidation of ethylene was comparatively studied over three different catalysts, including a commercial 14.86 wt.% Ag/alpha-Al2O3, a bimetallic 1.41 wt.% Cu-17.16 wt.% Ag/SrTiO3, and a 0.32 wt.% Sn-promoted on 1.39 wt.% Cu-17.16 wt.% Ag/SrTiO3 catalyst. The best diluent gas for ethylene epoxidation was CH4 balanced with He over all catalysts. CH4 enhanced C2H4 conversion while He provided both high EO selectivity and yield. Thus, the 35% CH4 balanced with 53% He diluent gas provided the highest synergistic effect toward the ethylene epoxidation reaction, while also leading to the lowest level of coke formation. (C) 2014 Elsevier B.V. All rights reserved.