Anion radicals of o- and p-bis(trimethylsilyl)benzenes were found by ESR spectroscopy to form the corresponding triplet dimers in 2-methyltetrahydrofuran (MeTHF) glass matrices at low temperatures. The temperature dependence of the signal intensity of a triplet dimer has shown that the dimer has the triplet ground state. The spin-spin distances estimated with the D values increased with the increasing size of alkali metal countercations, indicating that the triplet dimer has a pi(-)-M+-pi(-) type sandwich structure. The stability of the triplet dimer depended remarkably on the coordination ability of the solvent; no triplet dimer was observed in tetrahydrofuran (THF). (C) 2002 Elsevier Science B.V. All rights reserved.