Reactions of gas-phase ozone with alkene-terminated alkanethiol self-assembled monolayers on Au are explored using reflection-absorption infrared spectroscopy (RAIRS). The experiments are performed by using a directional doser to control the ozone flux onto the surface and in situ high vacuum techniques to eliminate reactions with atmospheric contaminants. We find that reactions between ozone and the C=C terminal group proceed through the formation of a carboxylic acid moiety that subsequently converts to an interchain carboxylic acid anhydride.