Iodine scavenging techniques have been used to examine the role of the cyclooctyl radical in the radiolysis of cyclooctane with gamma-rays and with 5-20-MeV helium ions. In gamma-radiolysis about 70% of the total yield of 6.6 cyclooctyl radicals/100 eV are scavenged with 10(-4) M iodine, which agrees well with other studies on cycloalkanes that show most of the radicals produced in these systems react in the bulk medium at times longer than 1 mu s. However, it is found that 2.5 radicals/100 eV (38%) are produced by H atom precursors as compared to a value of 1.5 cyclohexyl radicals/100 eV (25%) in cyclohexane. With 10-MeV helium ions (average LET of 106 eV/nm), only 8% of the cyclooctyl radicals survive longer than a few microseconds due to the increased initial radical concentration in the helium ion track. The yield of the cross-bridged product bicyclo[3.3.0]octane (pentalane) was found to be independent of iodine concentration up to 0.03 M with both types of radiation. However, the pentalane yield found with 10-MeV helium ions was only one-third of that found in gamma-radiolysis. The most likely reason for this result is the decreased yield of singlet-state formation due to the enhanced probability of cross combination reactions of electron-cation pairs in the high-density region of the helium ion track.