Rate coefficients have been measured for the gas-phase reactions of methyl, ethyl, n-propyl, isopropyl, tert-butyl, and neopentyl chlorides and bromides with the following set of nucleophiles, listed in order of decreasing basicity: HO − , CH 3 O − , F − , HO − (H 2 O), CF 3 CH 2 O − , H 2 NS − , C 2 F 5 CH 2 O − , HS − , and Cl − . For methyl chloride the reaction efficiency first falls significantly below unity with HO − (H 2 O) as the nucleophile and for methyl bromide with HS − as the nucleophile; in both cases the overall reaction exothermicity is about 30 kcal mol −1 . Earlier conclusions that these halides react slowly with stronger bases are shown to be in error. In the region where the rates are slow oxygen anions react with the alkyl chlorides and bromides by elimination while sulfur anions of the same basicity react by substitution. This difference is due to a slowing down of elimination with the sulfur bases; sulfur anions show no increased nucleophilicity as compared to oxy anions of the same basicity