The present study aims to identify cross-reactions of methane and dimethyl ether (DME) at low temperatures. A weak flame in a micro flow reactor with a controlled temperature profile (MFR) for a stoichiometric methane/DME/air mixture with 90% CH4 and 10% DME (hereinafter expressed as 90%/10% CH4 /DME) was used and analyzed. The weak flame structure was investigated by chemiluminescence observation and laser-induced fluorescence (LIF) measurements of OH and CH2O. Measured chemiluminescence and fluorescence intensity profiles were compared to computations with five detailed chemical kinetic mechanisms. Both experimentally and computationally obtained results showed the weak flame position of 90%/10% CH4/DME shifted to lower temperature in MFR compared to that of 100% CH4 reported earlier, indicating methane reactivity enhancement by the small DME addition. CH2O-LIF showed extensive formation of CH2O at low temperatures. Each mechanism predicted different levels of CH2O formation. The four mechanisms among five which include intermediate-temperature oxidation chemistry for methane , CH3 -> CH3O2 -> (CH3O2H) -> CH3O -> CH2O, predicted two-staged formation of CH2O at 650 and 730 K, whereas another mechanism predicted single-stage formation of CH2O at 650 K. Rate-of-production analysis revealed that OH radicals formed from low-temperature oxidation of DME initiate H-atom abstraction from methane; then intermediate-temperature oxidation chemistry for methane proceeds even at low temperatures. The contribution of intermediate-temperature oxidation chemistry for methane to CH2O formation at 730 K is stronger than that at 650 K, causing two-stage formation of CH2O. (C) 2020 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute.