The National Institute of Standards and Technology (NIST) detailed reaction mechanism of methane combustion was optimized based on a flow reactor experiment to obtain syngas (H-2 + CO). The experimental methane partial oxidation was conducted with pre-mixed gas in a flow reactor. Specifically, 0.2% methane and 0.1% oxygen were diluted with 99.7% argon, restraining the exothermic effect. The experiment was conducted from 1223 K to 1523 K under pressure. Through a comparison of the experimental results with calculated values, the NIST mechanism was selected as a starting point. Rate coefficients of O + OH = O-2 + H, CH3 + O-2 = CH3O + O, and C2H2 + O-2 = HCCO + OH were replaced with results from other studies. The replaced rate coefficient for CH3 + O-2 = CH3O + O was again optimized, within its reported uncertainty of 3.16, based on the experimental results of this study. The revised value of the rate coefficient for CH3 + O-2 = CH3O + O was k(37) = 7.92 x 10(13) x e((-31400/RT)). The optimized mechanism showed better performance in predicting the results of other studies, as well as this study. The optimization reduced the RMS error for the results of this study from 6.7 to 1.18. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.