Despite the fact that benzene (Bz) and pyridine (Py) are probably the most common and extensively studied organic molecules, the observation of a covalent adduct in the ionized benzene/pyridine system has never been reported. This Article reports the first experimental and theoretical evidence of a covalent (Bz center dot Py)(center dot+) adduct that results from the reaction of Bz(center dot+) with pyridine or Py center dot+ with benzene. These reactions are studied using mass-selected ion mobility, chemical reactivity, collisional dissociation, and ab initio calculations. The (Bz center dot Py)(center dot+) adduct does not exchange ligands with Bz to form BZ(2)(center dot+) or with Py to form (Py)(2)H+ despite the strong bonds in these homodimers. The thermochemistry then suggests that the (Bz center dot Py)(center dot+) heterodimer is bonded covalently with a bonding energy of >33 kcal/mol. Correspondingly, ab initio calculations identify covalently bonded propeller-shaped isomers of (Bz center dot Py)(center dot+) with bonding energies of 31-38 kcal/mol, containing a C-N bond. The mobility of the (Bz center dot Py)(center dot+) adduct in helium is consistent with these covalent dimers. As to noncovalent adducts, the computations identify novel distonic hydrogen-bonded complexes (C5H5NH+center dot C6H5 center dot) where the charge resides on one component (PyH+), while the radical site resides on the other component (C6H5 center dot). Collisional dissociation suggests that the covalent and distonic dimers may interconvert at high energies. The most stable distonic (C5H5NH+center dot C6H5 center dot) complex contains a hydrogen bond to the phenyl radical carbon site with a calculated dissociation energy of 16.6 kcal/mol. This bond is somewhat stronger than the NH+center dot pi hydrogen bonds of PyH+ to the pi system of the phenyl radical and of the benzene molecule. For this NH+center dot pi bond in the PyH+center dot Bz dimer, the measured binding energy is 13.4 kcal/mol, and ab initio calculations identify two T-shaped isomers with the NH+ pointing to the center of the benzene ring or to the negatively charged C atoms of the ring. In contrast, the more stable proton-bound PyH+center dot Py dimer contains a linear NH+center dot center dot center dot N hydrogen bond. The formation of the (benzene/pyridine)(center dot+) adduct may represent a general class of addition reactions that can form complex heterocyclic species in ionizing environments.