The inhibitory effects of CH3CI on an atmospheric pressure fuel-lean CH4/air premixed laminar flat flame are investigated through stable species (water-cooled quartz microprobe and gas chromatography) concentration measurements and detailed chemical modeling. Varying degrees of CH3CI in the feed (at a fixed π) result in consistently higher observed CO/CO2molar concentration ratios at all heights-above-burner.Modeling of these flames with detailed elementary reaction mechanisms yields predicted stable species concentration profiles which are consistent with the experimental observations. Rate-of-production analyses derived from the modeling indicates that the CI atom is a dominant radical in the CH3CI/CH4flames. Forfuel-lean conditions, the inhibitory effect results primarily from the slowdown of CO burnout by OH due to depletion of OH by HCI, which is generated primarily from fast H abstractions by CI.