The gas-phase reactions of the NO 3 radical with 2-methylthiophene, 3-methylthiophene, and 2,5-dimethylthiophene have been studied, using relative and absolute methods at 298 K. Determination of relative rate was performed using Teflon collapsible bag as the reaction chamber and gas chromatography as the analytical tool. For the absolute method, experiments were carried out using fast-flow-discharge technique with detection of NO 3 by laserinduced fluorescence. The temperature dependence was studied by the absolute technique for the reactions of NO 3 with 2-methylthiophene and 3-methylthiophene in the range 263-335 K.The proposed Arrhenius expressions for the reaction of the nitrate radical with 2-methylthiophene and 3-methylthiophene are k = (4 ± 2) × 10 −16 exp[−(2200 ± 100)/T]] cm 3 molecule −1 s −1 and k = (3 ± 2) × 10 −15 exp[−(1700 ± 200)/T]] cm 3 molecule −1 s −1 , respectively. C