Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe3+, Co2+, and Ni2+) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H-2) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H-2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H-2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni2+ used here). That is, the nickel precursor of Ni(OAc)(2) changes to Ni(H2O)(6)(2+) in neutral H2O and eventually transforms to Ni(OH)(2) nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)(2) to a defective structure [Ni-x(0)/Ni1-x(OH)(2)], which acts as the real catalytic species of H-2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (V-Ni) on the Ni(OH)(2) nanosheets enhance the adsorption and dissociation of H2O molecules to enhance the local proton concentration, while the Ni-0 clusters behave as H-2-evolution sites, thereby synergistically promoting the activity of H-2 photogeneration in alkaline media.