The contact angle relaxation of TiO2 surfaces is an important problem that must be understood, particularly for long-lasting hydrophilicity under dark conditions. The relaxation of sputtered anatase TiO2 thin films over a long time (similar to 22 days) in an atmospheric environment was observed using quantitative XPS analysis. A new peak was identified as H2O within a donor-acceptor complex at similar to 2.57 eV above the lattice oxygen peak. This donor-acceptor complex turns out to be a key factor for long lasting hydrophilicity, and our model is presented. Adventitious carbon contamination was not the main cause of the contact angle relaxation. Instead, samples with lower amounts of donor-acceptor complexes (I-DAC/I-bulk <=similar to 5%) underwent contact angle relaxation over time, and samples with a high density of donor-acceptor complexes (I-DAC/I-bulk >= similar to 10%) showed good hydrophilicity (contact angle <= 20 degrees) over 22 days. Larger amounts of basic Ti-OH relative to acidic OHbridge (ITi-OH/I-bridge >= 1) resulted in greater amounts of donor-acceptor complexes (I-DAC/I-bulk >= similar to 10%). Thus, basic Ti-OH groups interact with H2O by forming a strong electrostatic donor-acceptor complex, leading to long-lasting hydrophilicity. Indeed, TiO2 was transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment by forming sufficient Ti-OH groups and H2O molecules in the donor-acceptor complexes. Contact angle relaxation is closely related to the interactions between water molecules and the TiO2 surface in the dark. It is suggested that the relaxation depends on the number of electrostatic donor-acceptor complexes. This study provides new insight by linking theoretical studies with the experimental contact angle at the TiO2 surface in an ambient environment and is the first study that provides the presented relaxation mechanism.