The rate constants for the OH + isoprene and OH + ethylene reactions have been measured in He with 10% O-2 (P = 2-6 Torr) and over the temperature range 300-423 K using a discharge-flow system coupled with laser-induced fluorescence. The measured rate constants for the OH + ethylene reaction are in good agreement with previously reported values. The termolecular rate constant (k(0)) at the low-pressure limit for the OH + ethylene reaction was determined to be (2.62 +/- 0.25) x 10(-29) cm(6) molecule(-2) s(-1) at 300 K where the uncertainty represents 2 standard errors. An Arrhenius expression of k(0) = (4.9 +/- 0.2) x 10(-31) exp[(1210 +/- 130)/T] cm(6) molecule(-2) s(-1) was obtained from a weighted linear least-squares fit of the k(0) data versus temperature. The observed negative activation energy (-E-a/R) is larger than the currently recommended value. Unlike the OH + ethylene reaction, the rate constant fur the OH + isoprene reaction is independent of pressure between 2 and 6 Torr at 300 K. The measured rate constant of (1.10 +/- 0.04) x 10(-10) cm(3) molecule(-1) s(-1) at 300 K and 2 Torr agrees well with those measured at higher pressures. However, the rate constant for the OH + isoprene reaction begins to show a pressure dependence at temperatures of 343 K and higher. At 343 K, the termolecular rate constant was observed to be (10.50 +/- 1.57) x 10(-27) cm(6) molecule(-2) s(-1). An Arrhennius expression of k(0) = (9.3 +/- 5.3) x 10(-29) exp[(1560 +/- 230)/T] cm(6) molecule(-2) s(-1) was obtained from a weighted linear least-squares fit of the K-0 data versus temperature. The negative activation energy for the OH + isoprene reaction is similar to that observed for the OH + ethylene reaction.