We present the first direct study on the thermal unimolecular decomposition of allyl radicals. Experiments have been performed behind shock waves, and the experimental conditions covered temperatures ranging from 1125 K up to 1570 K and pressures between 0.25 and 4.5 bar. Allyl radicals have been generated by thermal decomposition of allyl iodide, and H-atom resonance absorption spectroscopy has been used to monitor the reaction progress. A marked pressure dependence of the rate constant has been observed which is in agreement with the results from a master equation analysis. However, our experimental results as well as our Rice-Ramsperger-Kassel-Marcus calculations seem to contradict the results of Deyerl et al. (J. Chem. Plqs. 1999, 110, 1450) who investigated the unimolecular decomposition of allyl radicals upon photoexcitation and tried to deduce specific rate constants for the unimolecular dissociation in the electronic ground state. At pressures around 1 bar we extracted the following rate equation: k(T) = 5.3 x 10(79)(T/K)(-19.29) exp[(-398.9 kJ/mol)/RT] s(-1). The uncertainty of the rate constant calculated from this equation is estimated to be 30%.