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Operando NMR combined with the DFT calculations has been exploited to probe the crucial role of the bulk H 2 O in the CH 3 OH heterogeneous photochemistry processes on the surface of rutile-TiO 2. • Bulk H 2 O can significantly affect the CH 3 OH reforming products. • The participation ways of bulk H 2 O can vary from H 2 O molecule to H-atom. • The productivities of CH 3 OH reforming products vary with H 2 O concentration. • Operando NMR can be used to probe the kinetics of the CH 3 OH photocatalysis. Solid-liquid heterogeneous catalysis reactions play key roles in industrial catalytic reactions, throughout pollution control, fuel cell, methanol reforming and so on. But, few operando techniques have been performed to track the complex reaction processes, especially those in bulk liquid environment containing two or more liquid reactants, thus leaving a gap in our understanding of the reaction mechanisms deep into the working conditions. Here, we demonstrate an approach, using operando nuclear magnetic resonance (NMR) to probe the dynamics and kinetics of the methanol heterogeneous photochemistry processes in liquid condition with and without bulk water (H 2 O). It shows that not only can bulk H 2 O affect the types of methanol reforming products, but also its participation way can vary from H 2 O molecule to H-atom. Furthermore, the productivities of the main products - methylene glycol (HOCH 2 OH) and hemiacetal (CH 3 OCH 2 OH) – show different dependency on the H 2 O concentration, which is further supported by the density functional theory calculations (DFT). The operando NMR method opens up exciting opportunities for the mechanism studies of heterogeneous catalysis reaction in bulk liquid. [ABSTRACT FROM AUTHOR]