The gas-phase reaction of chlorine atoms with isoprene was studied in 1-20 Torr of N-2 or Ar diluents using pulsed laser photolysis/vacuum UV laser-induced fluorescence and in 1-700 Torr of N-2 diluent using smog chamber relative rate techniques. There was no discernable effect of total pressure or nature of the diluent gas indicating that even in 1 Torr of argon diluent the reaction is near its high pressure limit. A value of k(CI + isoprene) = (3.44 +/- 0.32) x 10 (10) cm(3) molecule (1) s (1) was derived and is recommended for use in atmospheric models. The molar HCI yield in 700 Torr of N-2 diluent was 12.6 +/- 3.0%. (C) 2009 Elsevier B. V. All rights reserved.