We present a theoretical method to analyze photoinduced hydrophilic conversion processes of TiO2 surfaces, and apply it to recent experimental results of Sakai et al. (J. Phys. Chem. B 2003, 107, 1028). According to Young's theory, the cosine of the contact angle is a function of the interfacial energy between the solid and the liquid, which in turn changes with the surface fraction of hydrophilic regions. As the photoreaction proceeds, the surface fraction of hydrophilic regions increases, which lowers the interfacial energy between the solid and the liquid, resulting in the lowering of the contact angle. Therefore, the cosine of the contact angle is more appropriate to extract both the rates of hydrophilic conversion processes and the rates of back-processes, compared with the reciprocal of the contact angle proposed empirically by Sakai et al. Our method provides a quantitative way to analyze the kinetics of both photoinduced hydrophilic conversion processes and the back-processes to the hydrophobic states during long-term storage in the dark.