Display omitted •Ag particle size distribution over Ag/α-Al2O3 follows a lognormal distribution.•A Gaussian rate function explains particle size effects in ethylene epoxidation.•Ag particle size distributions centered ∼ 140 nm facilitate high epoxidation rates. The post-reaction Ag surface area distribution of 16 Ag/α-Al2O3 ethylene epoxidation catalysts with varying Ag weight loadings (6.9 – 35 wt%) containing different distributions of Ag particle sizes was correlated with measured ethylene oxide (EO) rates in presence of 3.5 ppm ethyl chloride via a particle size dependent Gaussian rate function. The Gaussian rate function model differs from a description based on average particle size as it accounts for the effects of the breadth of particle size distribution as well as the population of different particle sizes that constitute the Ag particle distribution on measured EO rates. This description of particle size effects in ethylene epoxidation catalysis facilitates accurate prediction of EO rates over a wide range of particle sizes (5–400 nm) and suggests that very small (5–50 nm) and very large Ag particles (>250 nm) have low EO rates (<3 μmol gAg-1 s−1). A narrow Ag particle size distribution centered around ∼130–150 nm would enable operation of ethylene epoxidation with high EO rates and EO selectivity.