Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO2, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278-319 K and 100-503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn- MVKO was measured to be k(uni) = 70 +/- 15 s(-1) (1 sigma uncertainty) at 298 K with negligible pressure dependence. In addition, k(uni) increases from ca. 30 s(-1) at 278 K to ca. 175 s(-1) at 319 K with an effective Arrhenius activation energy of 8.3 +/- 2.5 kcal mol(-1), k(uni)(T) = (9.3 X 10(7))exp(-4200/T) s(-1). Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.