The presence of physisorbed water during the silanization of silica is well-known for strongly influencing the silane reactivity at the interface. In this work, the reactivity of bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT), widely used in the rubber industry, on hydrated precipitation silica was investigated by time-resolved operando FTIR spectroscopy and chemometrics. The predominating reaction scheme is elucidated in conditions representative of industrial mixing process at the molecular scale. Based on multivariate curve resolution analysis and a quantitative kinetic model, it is shown that TESPT chemisorption is governed by two competitive reaction routes both producing only ethanol in the gas phase: (i) direct grafting reaction between an ethoxy moiety and a silica surface silanol and (ii) silane hydrolysis followed by co-condensation with a vicinal silane species. Reactions involving silanol–silanol condensation with production of water were not found to be significant. Although several types of water coexist on hydrated silica surface, it is demonstrated that strongly adsorbed monolayer water (Ea = 44 ± 2 kJ mol–1), which represents 5% w/w of the physisorbed water, is of primary importance in the hydrolysis reaction. Its relative surface concentration with respect to the amount of physisorbed silane is strongly correlated to the final ratio between co-condensation and grafting.