Size-dependent surface binding forms of the carboxylic group on the surface of TiO2 nanoparticles were investigated by a surface-binding transient probe molecule all-trans-retinoic acid (ATRA), where the excited triplet-state probe molecule generated by a photoinduced interfacial charge recombination for the adsorbed monolayer acts as a reporter for the different surface binding forms. Different TiO2 nanoparticles of varying size-that is, 6, 0.8-1.4, and 0.7 nm-were prepared, and their physical properties were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS). The transient absorbance difference spectra of the ATRA/TiO2 reveal that, when the size changes from 6 nm to < 1.4 nm, the simple adsorption forms (physical adsorption and hydrogen bonding form) decrease from 36% to 3%, whereas chemical binding forms increase from 74% to 97%. Such a size-dependent effect is attributed to the variation in the relative compositions of surface Ti atoms of different coordination states, which, in turn, form undercoordinated surface defect sites when the size varies at nanoscale. Referring to the results of the size-dependent coordination states of the surface Ti atoms from X-ray absorption near-edge spectroscopy (XANES) [T. Rajh et al., J. Phys. Chem. B 1999, 103, 3515-3519], the change of relative compositions for the 6-, 5-, and 4-fold coordinated surface Ti atoms, with respect to the particle size, can be rationalized. The dependency of the coordination state of the surface Ti atoms on the particle size provides a quantitative basis for tuning the compositions of the surface Ti atoms of different coordination state, as well as the surface binding forms by the control of the particle size. Surface modifications utilized in the Gratzel cells were discussed, in view of the surface binding and the coordination state of the surface Ti atoms.