The gas-phase reactivity of a set of halocarbocations, +CH2X (X = Cl, Br, or 1), (+CHXX2)-X-1 (X-1, X-2 = F, Cl, or Br), and +CX3 (X = F or CI), with four prototype aromatic compounds (benzene, furan, pyrrole, and pyridine) was investigated via double- and triple-stage mass spectrometry and compared to that of the simplest +CH3 carbocation. A rich chemistry is observed, and the reaction channels are greatly influenced by the number and type of halogen substituents (X), the strength of the C-X bonds, the nature of the aromatic compound, and the relative stabilities of the carbocation products. [Ar-CH2](+), [Ar-CHX](+), or [Ar-(CXX2)-X-1](+) functionalization of the relatively inert aromatic Ar-H bonds is the main reaction channel observed. A structure-specific "methylene by hydride exchange" reaction with toluene and B3LYP/6-311G(d,p) calculations indicate that the benzylium ion and the 2-furanylmethyl cation are formed in the [Ar-CH2](+) functionalization of benzene and furan, respectively. Kinetic isotope effects for the [Ar-CHX]l functionalization using naturally occurring halogen isotopes (Cl-35/Cl-37 and Br-79/Br-81) were measured. Using halogen-mixed halocarbocations (+CHXX2)-X-1, we evaluated the intrinsic competition for either the [Ar-CHX1](+) or [Ar-CHX2](+) functionalization. In reactions with pyridine, no Ar-H functionalization occurs and either proton transfer, N-addition, or net [CH2](+.) transfer due to the loss of X* from the nascent adducts is observed. Structural characterization of product ions was performed by on-line collision-induced dissociation or ion/molecule reactions, or both, and when possible by comparison with authentic ions.1083470097020