The isomerization and decomposition of 3,5-dimethylisoxazole were studied behind reflected shocks in a pressurized driver single pulse shock tube over the temperature range 880-1050 K and overall densities of similar to 2.5 x 10(-5) mol/cm(3). The main thermal reaction of 3,5-dimethylisoxazole is an isomerization to 2-methyl-3-oxobutyronitrile (CH3COCH(CH3)CN). Acetonitrile, which is the main decomposition product in the system, is obtained from 3,5-dimethylisoxazole by a unimolecular process : 3,5-dimethylisoxazole --> CH3CN + CH3CHCO. The unimolecular decomposition of the thermally excited isomer, (CH3COCH(CH3)CN)(#) --> CH2CH2CN + CH3CO, is assumed to be the source for a large concentration of free radicals and for carbon monoxide. Thus, ethane, methane, and carbon monoxide are the major products among the species without nitrogen. It is believed that hydrogen cyanide, acetaldehyde, and 2-buten-3-one, which are found in the postshock mixtures, are obtained by a four-center unimolecular elimination from 2-methyl-3-oxobutyronitrile from both the thermally excited state (low temperatures) and a state of thermal equilibrium (high temperatures). The isomerization reaction, which involves cleavage of the N-O bond and migration of a methyl group from position 3 to 4 in the ring, is a first-order process with a rate constant given by k(1st) = 10(15.90) exp(-61.5 x 10(3)/RT) s(-1) where R is expressed in units of cal/(K mel). The overall decomposition of 3,5-dimethylisoxazole in terms of a first-order rate constant is given by k(total) = 10(15.57) exp(-59.5 x 10(3)/RT) s(-1). A reaction scheme containing 26 species and 32 elementary reactions was constructed to describe the overall mechanism of 3,5-dimethylisoxazole decomposition and isomerization.