Dimerization of the radical anions of aromatic acetylenes (A(.-)) such as diphenylacetylene and its derivatives with substituents on the benzene ring (1(.-)), 1,4-diphenyl-1,3-butadiyne (2(.-)), and intramolecular dimer model compounds having two diphenylacetylene chromophores linked by several methylene chains (3(.-)) has been studied with pulse radiolysis of A in solutions at room temperature and gamma-radiolysis in rigid matrices of A at 77 K. The transient absorptions of A(.-) decayed with the formation of new bands assignable to the dimer radical anions of A(.-) and A. Because the decay and formation depend on the concentration of A, the bimolecular rate constants of k(b) = 7.3 x 10(6) to 6.6 x 10(7) M(-1) s(-1) were estimated for the intermolecular dimerization at room temperature. The spectral changes were also observed upon warming of 77 K rigid matrices of A(.-). It is suggested that A(.-) dimerizes with A through the formation of one C-C bond between two sp carbons, giving sigma-type dimer radical anions (sigma-A(2)(.-)) with a diene-type structure. Absorption spectra similar to those of 1(.-) were initially observed in 3(.-) generated by the radiolyses but changed to those assignable to the intramolecular dimer radical anions similar to sigma-1(2)(.-). The yield of the dimer radical anions of 3(.-) with a tetramethylene chain was the largest among the dimer radical anions with several methylene chains.