The major products produced in radiolysis of cyclooctane with 1-15 MeV protons, 10-30 MeV carbon ions, and 20-30 MeV oxygen ions were investigated in the absence and presence of iodine radical scavenger and compared with the gamma and 5-20 MeV helium ion radiolysis published in a previous work (J. Phys. Chern. 1994, 98, 8014). While almost 70% of the cyclooctyl radicals survive until a few microseconds in gamma-radiolysis, the yields with carbon ions and oxygen ions are about an order of magnitude lower. It appears that all of the hydrogen atoms produced in gamma-radiolysis react by hydrogen atom abstraction from the medium molecules, whereas in the heavy-ion tracks enhanced intratrack reactions eliminate most of the hydrogen atoms before such reactions can occur to any significant extent. The yield of products derived from the excited singlet state steadily decreases with the linear energy transfer (LET), and in carbon and oxygen ion radiolysis its yield is less than 20% of that found in gamma-radiolysis. Product formation through radical mechanisms first slightly increases with particle LET until about 50 eV/nm, and then the total yields of all products decrease to half of that found in gamma-radiolysis. Much of the change in product yields with increasing LET can be explained by a radical mechanism, but it also appears that there is a change in initial radical formation due to a shift toward an increased probability of triplet excited-state formation over that of singlet excited states.