The decomposition of 4-methylpyrimidine was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 1160-1330 K at overall densities of similar to3 x 10(-5) mol/cm(3). A plethora of decomposition products, both with and without nitrogen was found in the post-shock mixtures. They were HCN, CH3CN, C2H3CN, pyrimidine, C2H5CN, cis- and trans-CH3CH=CHCN, CH2=CHCH2CN, CH=C-CN, and C2N2, among the nitrogen containing products, and CH4, C2H2, CH3C=CH, C2H6, CH2=C=CH2, and C2H4 as products without nitrogen. It is suggested that the decomposition of 4-methylpyrimidine has two major initiation steps. (1) An ejection of a hydrogen atom from the methyl group that is connected to the ring, C4H3N2-CH3 --> C4H3N2-CH2. + H-., and (2) split of the methyl group from the ring, C4H3N2-CH3 C4H3N2 -CH3 --> C4H3N2. + CH3. .The ejection of H atom from the ring is much slower and does not contribute much to the total rate. The H atoms and methyl radicals initiate a chain mechanism by abstraction of an Il atom from the methyl group and by dissociative attachment of an H atom and removal of a methyl group from the ring. The pyrimidyl (C4H3N2.) and methylene pyrimidine (C4H3N2-CH2.) radicals decompose by ring cleavage followed by breakdown of the open-ring radical. The total decomposition of 4-methylpyrimidine in terms of a first-order rate constant is given by k(total) = 10(15.37) exp(-83.5 x 10(3)/RT) s(-1). A reaction scheme containing 33 species and 91 elementary reactions was constructed to account for the observed product distribution. A discussion on the decomposition mechanism is presented.