The thermal unimolecular reaction of pyruvonitrile diluted in Ar has been studied behind reflected shack waves over a temperature range 1014-1300 K, with a total density of ca. 1.0 x 10(-5) mol/cm(3). The reaction was monitored by means of time-resolved vacuum-UV absorption and IR emission. Also, ab initio molecular orbital (MO) calculations were carried out in order to formulate a reaction mechanism. From the gas chromatographic analysis of the shock-heated gas and from ab initio MO calculations, it is suggested that the isomerization to acetyl isocyanide occurs under the present experimental conditions. It is estimated that the difference of enthalpies between these species is 5 kcal/mol. The first-order rate constants are expressed as k(f) = 10(11.49) exp(-49.3 kcal mol(-1)/RT) s(-1) for the isomerization of pyruvonitrile to acetyl isocyanide and k(r)= 10(11.36) exp(-44.3 kcal mol(-1)/RT) s(-1) for the reverse reaction.