For the first time, we show that chlorine has a dramatic effect on the selectivity for olefin epoxidation promoted by Au. Chlorine coadsorbed with oxygen on Au(111) inhibits combustion, formation of organic acids, and deposition of residual carbon. The temperature required for formation of styrene oxide is also lowered relative to O-covered Au(111), indicating faster kinetics for epoxidation. This work provides strong evidence that chlorine would be an effective promoter for enhancing the performance of Au-based selective oxidation catalysts.