Studies have shown that environmentally and biologically relevant coatings on nanoparticle (NP) surfaces can significantly alter the physicochemical properties (e.g. dissolution and aggregation) of particles yet there remain some questions on how these coatings impact reactivity. In this study, we investigated molecular-level details of surface adsorption and surface reactivity of titanium dioxide (TiO₂) NPs using in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) in the presence of bovine serum albumin (BSA) protein and fulvic acid (FA), which were selected as representative biologically and environmentally relevant molecules. Our results show that both BSA and FA adsorb strongly and irreversibly onto TiO₂ NP surfaces at neutral pH and these surface coatings impact the photochemical behavior of TiO₂. In particular, we show large differences in the formation of reactive oxygen species (ROS) for coated compared to uncoated TiO₂ NPs, as well as differences between the two different coatings. In the absence of any coatings, the photooxidation of solution phase sodium benzoate (BA) to hydroxyl benzoate (major product) is observed. However, this reaction is completely inhibited when TiO₂ is coated with BSA and partially inhibited when TiO₂ is coated with FA. Additionally, we found that BSA can strongly scavenge ROS generated upon irradiation by quenching the formation of electron–hole pairs. In contrast, the behavior of FA shows photoinduced hydrophilicity of the TiO₂ coated surface and the generation of ROS, although less than that of the uncoated TiO₂ NPs. Overall, these results show that the formation of ROS from TiO₂ NPs coated by BSA and FA is reduced. Overall, this study provides insights into the impacts of environmentally and biologically relevant coatings and how they may modify the reactivity of the NPs in the environment. Furthermore, the implications of this study extend to understanding the potential reduced toxicity and impacts of TiO₂ NPs with coatings in natural and human-impacted ecosystems.