Promoters are ubiquitous in heterogeneous catalysis. The great majority of catalysts developed for commercial use are modified by promoters to enhance the yield of the desired product. Here, we report an investigation of the promotion effect using near ambient pressure X-ray photoelectron spectroscopy, whereby we directly observe a promoter modifying the active sites of a working catalyst. For a silver catalyst under ethylene epoxidation, the increase in selectivity obtained by chlorine promotion is related to modifications in the balance between electrophilic and nucleophilic oxygen species, which constitute the catalyst active sites for the selective and unselective oxidation pathways. (C) 2014 Elsevier Inc. All rights reserved.