The isomerization of 2-methyl-4,5-dihydrofuran was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 805-1030 K and densities of approximately similar to 3 x 10(-5) mol/cm(3). Two isomerization products, acetylcyclopropane and 3-penten-2-one, are obtained in the isomerization. Acetylcyclopropane is formed in an irreversible process from 2-methyl-4,5-dihydrofuran. It further isomerizes, at higher temperatures, to cis- and trans-3-penten-2-one. At high temperatures where the conversion of 2-methyl-4,5-dihydrofuran is high, the main source for 3-penten-2-one is acetylcyclopropane. At lower temperatures 3-penten-2-one is formed mainly by a direct isomerization of 2-methyl-4,5-dihydrofuran. A small concentration of decomposition products, mainly methane and ethane, are also found in shock mixtures of 2-methyl-4,5-dihydrofuran, particularly at high temperatures. The Arrhenius relations for the three aforementioned processes are as follows : 2-methyl-4,5-dihydrofuran --> acetylcyclopropane, k(1) = 10(15.4) exp(-56.8 X 10(3)/RT) s(-1); 2-methyl-4,5-dihydrofuran --> 3-penten-2-one, k(2) = 10(15.7) exp(-63.6 X 10(3)/RT) s(-1); acetylcyclopropane --> 3-penten-2-one, k(3) = 10(14.4) exp(-58.3 x 10(3)/RT) s(-1), where R is expressed in units of cal/(K mol).