Dispersed TiO2 based photocatalysts are multifunctional and exhibit performances that result from the efficient interplay of various sequential and intermingled steps starting with photophysics and ending with mass transfer limitations at the particle interface. The nature and concentration of point defects acting as trapping and recombination sites for photogenerated charges underpin the materials photocatalytic activity. Defect characterization of particle systems requires an integrated characterization approach and should aim at quantitative relationships between power and energy of light and the number of reactive species. Here, we will briefly introduce into the characteristics of paramagnetic and optical fingerprints specific to trapped charges in dehydroxylated TiO2 particle systems. Recent efforts to infer the impact of defects as trapping sites on the photocatalytic performance from slow charge trapping processes will be reviewed. Moreover, it will be outlined how related photoexcitation studies can be extended to solid–liquid interface systems. Complementary photoelectrochemical experiments on particles in contact with aqueous electrolytes provide means for the quantification of charge trapping and, in addition, open the way to track proton-coupled electron trapping. Finally, we will address the issue of microstructural changes inside the particle ensemble that can occur during materials synthesis and processing. Related microstructural transformations of TiO2 nanoparticle ensembles typically involve the formation and breakage of particle–particle interfaces and can alter charge carrier recombination and trapping.