The thermal reactions of 2-methylindene diluted in argon were studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 1050- 1300 K and overall densities of similar to3 x 10(-5) mol/cm(3). A plethora of products resulting from decompositions, isomerizations, and ring expansion were found in the post shock samples. They were naphthalene as the product of five-membered ring expansion, 1- and 3-methylindene due to isomerizations, and CH4, CH4, C2H6, C2H2, benzene, toluene, ethylbenzene, styrene, phenylacetylene, and indene as the result of fragmentation. Very minute yields of some other compounds were also observed. Except for the isomerizations that take place from the reactant as a starting material, the production of all the other products involve free radical reactions. The initiation of the free radical mechanisms in the decomposition of 2-methylindene takes place via ejection of hydrogen atoms from spl carbons and dissociation of the methyl group attached to the ring. The H atoms and the methyl radicals initiate a free radical mechanism by abstraction of hydrogen atoms from sp(3) carbons and by dissociative recombination of H atom and removal of a methyl group from the ring. In addition to these dissociation reactions there are several breakdown processes that involve cleavage of the five-membered ring to produce both stable and unstable products. Tire ring expansion process that leads to the production of a high yield of naphthalene takes place only from a methylene indene radical intermediate rather than methylindene itself, whereas isomerizations take place from both the radical intermediates and the molecule. The total decomposition of 2-methylindene in terms of a first-order rate constant is given by: k(total) = 10(13.69)exp (-59.4 x 10(3)/RT) s(-1). A reaction scheme containing 34 species and 71 elementary reactions was composed and computer simulation was performed over the temperature range 1050-1300 K at 25 K intervals. The agreement between the experimental results and the model prediction for most of the species is satisfactory.