1,3,5-Trimethylenebenzene (TMB, C9H9) is a prototypical hydrocarbon triradical. CASPT2/ANO-L calculations confirm that TMB has a quartet ground state with a B-2(1)-(4)A(1)" gap of 12.4 +/- 2.0 kcal/mol; both B-2(1) and (2)A(2l) states are quasi-degenerate. C9H9- has nearly degenerate B-3(2), (3)A(1) and (1)A(1) states and C9H9+ nearly degenerate (3)A(1), (1)A(1) and B-3(2) states. Exocyclic protonation is favoured over ring-protonation; both processes induce a low-spin doublet ground state. Ionization energy, electron affinity, proton affinity are: IEa(TMB) = 7.5 +/- 0.2 eV, EA(TMB) = 22 +/- 2 kcal/mol, PA(C9H9) = 235 +/- 2 kcal/mol. (c) 2005 Elsevier B.V. All rights reserved.