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Operando NMR has been exploited to probe the effects of photon flux and wavelength on photocatalytic reforming of methanol. • Operando NMR technique can be used to study the CH 3 OH photochemistry process. • CH 3 OH oxidation reaction could be enhanced by photon flux. • Photo energy had different effects on CH 3 OH photoreactions for TiO 2 and Pd/TiO 2. • SPR of the metal cocatalyst can be observed at 380 nm for the Pd/TiO 2 sample. • Operando NMR may help know the photoreactions in practical working conditions. Photocatalytic decomposition of water for hydrogen generation is a promising way to obtain clean energy sources, and photon flux together with photon energy play particularly important roles in photocatalytic reactions. However, a study of their effects under real conditions is still limited, thus leaving a gap in our understanding. Herein, an operando NMR technique was proposed to systematically study the effects of photon flux and photon energy on TiO 2 and Pd/TiO 2 -based photocatalytic methanol reforming reactions by changing the irradiation wavelength from 350 to 400 nm. Experimental results showed that the productivity of the methanol oxidation reaction can be obviously influenced by photon flux for the pure TiO 2 and Pd/TiO 2 sample, but longer wavelengths (lower photon energy) can slow down the reaction. Meanwhile, on the Pd/TiO 2 catalyst a significantly enhanced activity in CH 3 OH oxidation was found at the irradiation with the wavelength of 380 nm, which activity in CH 3 OH oxidation is even higher than that observed under the irradiation with the wavelength of 365 nm. This enhanced oxidation activity can be attributed to the surface plasmon resonance of the cocatalyst. [ABSTRACT FROM AUTHOR]