Thermal decomposition of pyrazole, a five-membered nitrogen-containing heterocycle, has been studied using ab initio G3X theory and RRKM rate theory. The decomposition mechanism involves an intramolecular hydrogen shift to 3H-pyrazole, followed by ring opening to 3-diazo-1-propene and dissociation to vinylcarbene (CH2CHCH) + N-2. At 1 atm the calculated rate equation k [s (1)] = 1.26 x 10(50)T (10.699)e (41200/T) is obtained, which agrees with the results of flash vacuum pyrolysis experiments. The pyrazole decomposition product vinylcarbene is expected to rearrange to propyne, making pyrazole decomposition essentially thermoneutral. It is hypothesized that at high concentrations vinylcarbene could undergo a self-reaction to 1,3-and 1,4-cyclohexadiene. (C) 2009 Elsevier B. V. All rights reserved.