In the radiolysis of water vapor containing small concentrations of cyclohexane, the principal products which account for about 98% of all end products are found to be hydrogen, cyclohexene, and bicyclohexyl. Cyclohexene and bicyclohexyl yields were determined over a range of temperatures (70-200°C), total pressures (50-2400 torr), and total doses (0.15-2.0 Mrad). The disproportionation-combination ratio k;/kP for c -C~H I I radicals could be determined as 0.56 f 0.01 from the ratio of cyclohexene to bicyclohexyl yield. By using c-CgD12, the ratio kflkf for c-CgDII radicals is found to be 0.38 f 0.01. Comparison of the reactivity pattern of CgHll and CgDll radicals leads to (k;/kF)/(kf/kF) = 1.47 f 0.02. The corresponding values for the reactions of c-C~H11 with c-CgD11 were also determined.