The pyrolysis kinetics of acetonitrile dilute in argon has been studied in the temperature range 1400-2100 K at an average pressure of 12 atm in single-pulse shock tube experiments. The principal products are HCN, C2H2, CH4, and H-2, while the minor products include HCCCN, H2CCHCN, C2H4, and C4H2 The overall kinetics is successfully simulated by an 87 step kinetic model that accurately accounts for the temperature profiles of the major products and also provides an acceptable fit for the minor products. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio quantum chemical calculations carried out at CASSCF, CASPT2, and Gaussian-2 levels of theory. Several distinct reaction pathways were studied, whereby the geometries, vibrational frequencies, and energies of approximately 70 molecular species representing reactants, products, intermediates, and transition states were computed. The pyrolysis of acetonitrile is initiated by CH bond fission, forming a cyanomethyl radical. This reaction is the most sensitive one in the kinetic model. On the basis of sensitivity analyses of the model as well as ab initio calculations, the heat of formation of cyanomethyl has been revised as Delta(f)H(298)(0)(CH2CN) = 263 +/- 9 kJ mol(-1). The limiting high-pressure Value of the corresponding rate constant, as obtained by ab initio variational transition state calculations, is k(1 infinity) = 1.2 x 10(16) exp(-413 kJ mol(-1)/RT) s(-1), which is in good agreement with our extrapolated experimental measurement. A number of the observed products, including HCCCN and H2CCHCN, largely arise from the decomposition of succinonitrile, a key intermediate, that forms by the recombination of two cyanomethyl radicals.