Benzene and all mono-, di-, tri, tetra-, and pentafluoro-substituted derivs. thereof were equilibrated with the corresponding aryl anions in the gas phase. Perfect additivity of the substituent effects on the acidity was obsd. The basicities were diminished by 12, 6, and 4 kcal/mol, depending on whether the supplementary fluorine atom occupied the ortho, meta, or para position with respect to the deprotonation site. On the other hand, substituent effects on the rates of hydrogen/metal interconversion processes level off with increasing nos. of halogens. For example, while the free energy of activation of sec-butyllithium-promoted lithiation of fluorobenzene is at least 4 kcal/mol smaller than that of benzene, the difference was found to shrink to less than 2.5 kcal/mol when pentafluorobenzene and 1,2,3,4-tetraflorobenzene were compared. [on SciFinder (R)]