Most solar-energy conversion applications are based on trapping and transferring photoinduced electrons on oxide semiconductor nanoparticles, such as titanium dioxide, and broad UV-vis absorption (400~800 nm) and monotonic IR absorption (1100~3000 cm−1) signals have long been considered signatures of the electron-trapping state on titanium dioxide nanoparticles. Here we show that, under proton-free conditions and using iodide ions in acetonitrile as the hole scavenger, the intrinsic electron-trapping feature of titanium dioxide nanoparticles does not exhibit the characteristic UV-vis absorption and infrared absorption signatures. Further electron spin resonance studies identify the proton-free electron-trapping state as the lattice octahedral Ti_6c³⁺ species, differing from the traditional protonparticipating surface tetrahedral Ti_4c³⁺ species. Synchronized radiation ultraviolet photoelectron spectroscopy results also show that the internal electron-trapping state without protons has a larger Ti3d binding energy (1.8 eV) than the blue electron-trapping state (1.3 eV) that forms when protons participate and thus shows different electron transfer abilities. （Communications Chemistry. 2019, 88. DOI: s42004-019-0191-7）