Recoil bromine species produced by the systems 76,77Kr →76,77Br were reacted with a series of hydrocarbons in the gas phase over the density range 2 X 10-5 to 0.5 g/cm3. In the region below 100 atm differences in reactivity are related to differences in decay modes and recoil energies. In the low-density methane system an ion-molecule mechanism involving excited CH4Br+ is suggested. At higher pressures, the yields are explained in terms of the onset of “caging” reactions due to autoradiation effects. At high percent “inert” moderator, yields increased for CH4, above those of the pure methane system. Brominating complexes such as KrBr+ and ArBr+ are hypothesized to explain the results.