The cis-bent form of the acetylene radical anion (a(iso)(H) = 65 G) was observed following radiolysis of acetylene-loaded HZSM5 but not NaZSM5. The acetylene radical cation is not trapped by the matrix at 77 K and undergoes ion-molecule reactions to form multimer ions, including the benzene cation. Benzene is also formed by acid-catalyzed polymerization of acetylene on HZSM5. The evidence that acetylene (IP = 11.4 eV) is ionized places a new lower limit on the range of ionizable species in the radiolyzed zeolite. The energies, geometries, and hyperfine coupling constants were calculated for the acetylene, vinylidene, cyclobutadiene and butatriene radical anions and radical cations, and the benzene radical cation, using different levels of theory, including gradient-corrected density functional theory.