Abstract111 recent puhlicatiuiis I r o m this lahoratory. we have shown that the tragnientatiuii ol' photoexcited olel'inic moleculeh in the vacuum 1IV region leads mainly hi the cleavage ot'a ( ' 'I'he ally1 fragment hear> auav part 01 the excess energy ul the photoii. A t low pressure, this excited radical is capable t i t undergoing further decomposition. F r o m the pressure eftect. we were able to measure the Iir5t order rate constant for this secondary trapmentation. In this paper we shall use HRKM ( alculations in order to get a hetter idea un how the energy is distributed among the p r i m a r y l r a g m r n t x In cabes where cr. and 3 meLhallyl radicals were involved. the results show that iiii i i n p i r t a n t part ot'the excess energy is l w a t e t l in the methallyl fragment in t h e 7.1 a n d 7.6 r\' photolysis uf :]-methyl 1 butene. '-methyl. I -hutene. a i i d ( i.\-?-pentene.( * hind located in the 13 pusition r e l a t i t c t o the doublr bond