The thermal reactions of isodihydrobenzofuran (phthalan) were studied behind reflected shock waves in a single pulse shock tube over the temperature range 1050-1300 K and overall densities of similar to3 x 10(-5) mol cm(3). The total decomposition rate, expressed as a first-order rate constant, is 10(12.10) exp(-53.7 x 10(3)/RT) s(-1), where R is in given units of cal/(K mol). One isomerization product, o-tolualdehyde [k(isomerization)) = 10(16.50) exp (-80.5 x 10(3)/RT) s(-1)], and products;resulting from unimolecular cleavage of the furan ring were obtained under shock heating. Carbon monoxide and toluene are the products of the highest concentration. Isobenzofuran is obtained by a 1,6-H-2 elimination from the furan ring with a rate constant k = 10(13.78) exp(-70.0 x 10(3)/RT s(-1). In addition, benzene, ethylbenzene, styrene, ethylene, methane, and acetylene were found in the postshock mixtures. Trace quantities of allene and propyne were also found. In addition to II-atom ejection from the furan ring, it is believed that phthalan decomposes via two unimolecular reactions that involve cleavage of the furan ring. One reaction yields stable products, phthalan --> C6H5-CH3 + CO [k = 10(14.11) exp(-67.0 x 10(3)/RT) s(-1)], and one decomposition yields unstable intermediates which are responsible for the propagation of free radical reactions in the system, phthalan --> HCO. + C6H5-CH2. (k = 10(16.6) exp(-78.0 x 10(3)/RT) s(-1)). A reaction scheme containing 26 species and 49 elementary reactions was constructed, and computer modeling was performed in 25 K intervals over the range 1050-1300 K. The agreement between the experimental and the calculated yields is reasonable. Differences and similarities in the thermal behavior of isodihydrobenzofuran, dihydrobenzofuran, 2,3-dihydrofuran, and 2,5-dihydrofuran and the effect of fused benzene ring are discussed.