The deprotonation energies of benzene, chlorobenzene, all di-, tri-, tetrachlorobenzenes, and pentachlorobenzene have been detd. in the gas phase using a Fourier transform ion cyclotron resonance mass spectrometer. The values measured differ only slightly, though significantly, from the corresponding data for oligofluorobenzenes. The heavier halogen acidifies ortho-positions slightly less and meta-positions slightly more than fluorine does. Moreover, the contributions of three or more chloro substituents are not perfectly additive. In fact the accumulation attenuates the contributions somewhat. Quantum chem. calcns. at the MP2/6-311+G* level reproduce the gas-phase acidities fairly well, but reveal special effects when extended to exptl. not observable benzenides carrying the halogens at anion-remote positions. Competition expts. have been performed to assess the relative reactivity of nine oligochlorobenzenes towards sec-butyllithium in THF at - 100 DegC. An almost exact linear correlation between logarithmic rates and gas-phase acidities has been found. [on SciFinder (R)]