The deprotonation energies of benzene, fluorobenzene, all di-, tri-, and tetrafluorobenzenes, pentafluorobenzene, chlorobenzene, all di-, tri-, and tetrachlorobenzenes, and pentachlorobenzene have been calcd. at various levels of second-order Moller-Plesset and d. functional theory. Taking the previously detd. exptl. data as a benchmark, good agreement was achieved in the chloro series even with moderate computational effort, whereas more extended basis sets have to be used to obtain meaningful nos. in the fluoro series. Apparently, most extensive electron correlation is required to avoid artifacts caused by the proximity of non-bonding lone pairs at the carbanionic center and at the fluorine atoms. When two or more fluorine substituents were introduced in the same arom. ring, their individual effects (as defined by position-dependent acidity increments) proved to be perfectly additive in the entire series. In contrast, the acidifying effect of chloro substituents was found to level off when the no. of such halogens increases. Additivity or non-additivity of element effects cannot be ascertained after having merely compared the acidity of mono- and disubstituted substrates, but only after having moved to higher degrees of substitution. [on SciFinder (R)]