The formation rates and isomeric branching ratios of the four possible adducts arising from the Cl-isoprene reaction have been calculated using canonical variational transition state theory (CVTST). In addition, RRKM/master equation (ME) formalism is employed to investigate isomerization of the Cl-isoprene adducts. We found that there is no evidence for an energetic barrier for Cl addition to isoprene and the Morse potential well represents the energetics along the reaction coordinate, The results reveal the importance of terminal Cl addition to isoprene in partitioning of the final reaction products and in determining the reaction pathways. (C) 2002 Published by Elsevier Science B.V.