The aim of this paper is to analyze and discuss the kinetics of the pyrolysis and combustion of naphthenes. The primary propagation reactions of cyclohexane and methylcyclohexane are presented to extend the validity of a semi-detailed kinetic model for the pyrolysis and oxidation of hydrocarbons. Naphthenes are relevant species as reference components in liquid fuels and surrogate blends. A lumped approach is used to reduce the complexity of the overall scheme in terms of species and reactions. Particular attention is devoted to the role of the isomerization or internal abstraction of H atoms in competition with P-decomposition ones. Primary oxidation and decomposition reactions of the cyclohexyl radical are discussed to explain and justify this lumping procedure. The modeling predictions are compared with different sets of measurements. The validation of the low temperature oxidation mechanism of cyclohexane is based on the ignition delay times obtained both in the rapid compression machine at Lille and in closed vessels. Jet-stirred reactors at different pressures and stoichiometric ratios also confirm the reliability of the overall mechanism of oxidation. The comparisons between the model's predictions and the measurements relating to the pyrolysis and oxidation of methylcyclohexane in the Princeton turbulent flow reactor further support this extension of the kinetic scheme to naphthenes. Finally, the agreement with the oxidation experiments using mixtures of toluene + methylcyclohexane is a primary and simple example of the model's ability to deal with the combustion of real fuels or surrogate blends. (C) 2003 The Combustion Institute. All rights reserved.