Dimerization of a series of regioisomeric 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzene radical anions (bPEBs.−) substituted by various electron-donor and/or electron-acceptor groups was studied during the pulse radiolysis in N,N-dimethylformamide (DMF). The transient absorption of bPEB.− decayed with the formation of new bands attributed to the dimer radical anion of bPEB.− and bPEB (bPEB2.−). From the dependence of decay and formation rates on the concentration of bPEB, the bimolecular rate constants of kb=(1.2±0.1)×107 to (3.2±0.2)×108 M−1 s−1 were estimated. It is suggested that bPEB.− dimerizes with bPEB through the formation of a CC bond between two sp carbon atoms, to give σ-type dimer radical anion (σ-(bPEB)2.−) with a diene-type structure. The kb values of bPEB.− with bPEB changed with the substitution pattern of the phenylacetylene group on the central benzene ring and the various kinds of donor and/or acceptor group.