AbstractThe gas‐phase acidities of halogen‐substituted aromatic compounds have been determined in a flowing afterglowtriple quadrupole apparatus with use of the silane cleavage method developed by DePuy and co‐workers [C. H. DePuy, S. Gronert, S. E. Barlow, V. M. Bierbaum and R. Damrauer,J. Am. Chem. Soc., 111, 1968 (1989)]. In this method the relative yields of siloxide ion products produced in reactions of OH−with trimethylsilyl‐ or phenyldimethylsilyl‐substituted aromatic compounds are correlated with the difference in gas‐phase acidity of the accompanying neutral products. Acidities are reported for different ring‐positions in fluoro‐, chloro‐ and bromobenzene, chloro‐ and bromonaphthalene and benzyl chloride. Excellent precision is achieved in most cases, with assigned uncertainties less than 2–3 kcal/mol. Good agreement is obtained between the acidities determined with use of two different types of silane precursor. Halogen‐substitution increases the gas‐phase acidities of benzene and naphthalene by similar amounts (13–14 kcal/mol). The effects on different ring‐positions in benzene and naphthalene are shown to be primarily inductive in nature, falling‐off by a consistent 2.5–3.5 kcal/mol per bond separating the acidic site from the halogen‐bearing carbon in the chlorine and bromine‐substituted systems. Larger effects are evident in the positional acidities of fluorobenzene. Themetaandparaposition acidities of halobenzenes are shown to be linearly correlated with the acidities of the correspondingmetaandparahal