The rate constants for the OH + methyl vinyl ketone and OH + methacrolein reactions have been measured in 2-5 Torr of He and over the temperature range 300-422 K using discharge-flow systems coupled with laser-induced fluorescence and resonance fluorescence detection of the OH radical. The rate constant for the OH + methyl vinyl ketone reaction was found to be (1.73 +/-0.21) x 10(-11) cm(3) molecule(-1) s(-1) at 5 Torr. No significant pressure dependence was observed between 2 and 5 Torr at 300 K, but a pressure dependence of the rate constant was measured at temperatures between 328 and 422 K. At 328 K, the termolecular rate constant (k(0)) was measured to be (6.71 +/- 2.65) x 10(-28) cm(6) molecule(-2) s(-1). An Arrhenius expression of k(0) = (9.9 +/- 7.6) x 10(-30) exp[(1440 +/-300)/T] cm(6) molecule(-2) s(-1) over the temperature range 328-422 K was obtained from a weighted linear least-squares fit of the k(0) data versus temperature. Unlike the OH + methyl vinyl ketone reaction, a significant pressure dependence of the rate constant for the OH + methacrolein reaction was not observed between 2 and 5 Torr at T = 300-422 K. The measured rate constant was (3.23 +/- 0.36) x 10(-11) cm(3) molecule(-1) s(-1) at 2 Torr and 300 K, and exhibits a negative temperature dependence over the temperature range 300-422 K.