A possible reaction mechanism is suggested based on our studies and previous reports (Scheme 2). The first step is nitrosyl chloride NOCl is released into the gas-phase from the acidic solution of catalytic NaNO2 or NaNO3 with HCl at room temperature. Then chlorine radical Cl• and nitric oxide NO are formed from NOCl under visible-light. The effective wavelengths of LED light are 350~450 nm (Table ). In the gas-phase, CH4 is attacked by Cl• to give methyl radical CH3• and HCl, and CH3• combines with O2 to generate methyl peroxide species CH3-OO• subsequently, which is partially confirmed by an isotopic O test to find CH3 O species (Scheme S1). On the other hand, methyl chloride CH3-Cl could also be produced from the reaction of CH3• with NOCl, especially when O2 is insufficient and more NaNO2 is used (Table ). Meanwhile, another byproduct, formic acid HCO2H will be generated in the further reactions of CH3-OO• species readily . Fortunately, CH3-OO• can also react with NO to give CH3-OO-NO quickly in the gas-phase, and CH3-OO-NO could be isomerized to methyl nitrate CH3O-NO2 as a protected species to prevent over-oxidations happening under the same conditions, which is a key step for the product selectivity. It implies that the selectivity of methyl ester in the gas-phase depends on the ratio of NOCl/CH4/O2, and NOCl is from NO2 - or NO3 -with HCl in the liquid-phase. In the absence of HTFA, CH3O-NO2 was detected by H NMR analysis in the reaction as mentioned above. When CH3ONO2 was used instead of NaNO2, the reaction could still proceed very well (Table ). Finally, CH3O-COCF3 (MTFA) and HNO3 are produced by the transesterification of CH3O-NO2 with HTFA in the presence of HCl in the liquid-phase (Table ). NOCl is also regenerated from HNO3 and HCl in the solution. Actually, NOCl provides not only Cl• to cleave inert CH3-H bond but also NO to form CH3O-NO2 under visible-light at room temperature. Thus, it is a gas catalytic species for the total catalysis on both reactivity and selectivity. Moreover, a large amount of Cl2 generated rather than NOCl is not good to produce methyl ester selectively in the aerobic oxidation of methane, like under the reaction conditions for the visible-light driven oxidative chlorination of alkanes. Since the main reactions are carried out in the gasphase, including the breaking of CH3-H bond and the formation of CH3-O bond, it is unnecessary to demand good solubility of either CH4 or O2 in the liquid solvent. Some factors to hamper the formation of NOCl should be eliminated. For example, with the amount of MTFA increasing, the rate of transesterification of CH3O-NO2 with HTFA was slow down. Thus, more HCl added to enhance the acidity is beneficial to release HNO3 from CH3O-NO2 for the regeneration of NOCl in the continuous reactions. If there is another alkane with weaker C─H bonds such as ethane (BDE=420 kJ mol -1 ) 1 or cyclohexane (BDE=416 kJ mol -1 ) 37 in the reactor, a classical radical nitrosation occurs easily at a very low O2 concentration. Acetic acid or caprolactam was formed after oxime rearrangements in the liquid-phase, resulting in the loss of NO to stop the catalytic cycle.