Thermodynamic parameters such as species heats of formation and entropies may have a significant impact on the output of detailed kinetic models, but attention is generally only given to considering kinetic parameters in model development and optimisation. rn this paper, a method for comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of detailed kinetic models is described. This method employs kinetic and thermodynamic sensitivity analysis to define an impact factor for all parameters under consideration as the product of the sensitivity of a particular model output to the parameter and the uncertainty in the value of that parameter. Kinetic and thermodynamic impact factor analysis has been applied to the results of an experimental study of the interaction between NOx (ca. 0-200 ppm) and methane (ca. 40-1300 ppm) in the presence of oxygen (ca. 0-14%) in an atmospheric pressure how reactor, for temperatures ranging from 500 to 700 C. A detailed chemical kinetic model has been assembled to describe the measured profiles of CH4, CO, CO2, NO, and NO2. The predictions of the kinetic model are in very good agreement with the experimental data over the entire range of conditions. The NO- and methane-oxidation reactions promote each other, leading to NO2 and CO as the main products. The mechanism of methane oxidation was found to be very similar to the high-temperature methane oxidation mechanism, with respect to the intermediate species involved. However, the addition to NO provides a source of chain carriers at unusually low temperatures, since, in the presence of NO, the formation of methylperoxy radicals is no longer chain-terminating: CH3 O-2 + NO --> CH3O + NO2 dominates the fuel oxidation. This reaction is also the most important NO-oxidising step. Due to the high uncertainties of some thermodynamic species parameters, thermodynamic impact factors were found to be higher than kinetic impact factors over a wide range of conditions. From the comparison of experimental and modelling results, recommendations for the heat of formation of CH3O2 (Delta H-f(0.298) = 2.7 kcal.mol(-1)) and for the entropy of nitromethane (Delta S-0.298 = 65.7 cal.mol(-1).K-1) are derived.