<p>The selective oxidation of ethylene glycol to glycolic acid on the Pt₄, Pt₁₃, Pt₃₈, and Pt₅₅ clusters was investigated by using density-functional theory calculations. The calculated results imply that glycolic acid is preferentially generated through the dehydrogenation of ethylene glycol by OH to form HOCH₂CH₂O on the Pt₄, Pt₁₃, and Pt₃₈ surfaces, but that this process occurs directly without OH participation on the Pt₅₅ surface. The observed effect likely arises from the addition of OH, which modulates the electron density in the O atom of ethylene glycol, thereby affecting the cleavage of the O–H bond. Furthermore, the glycolic acid formation on the Pt_<i>n</i> clusters is limited by the <i>β</i>–H elimination of HOCH₂CH₂O to HOCH₂CHO, which exhibits the lowest energy barrier on the Pt₁₃ surface. It is because the <i>d</i>-band center of the Pt₁₃ cluster is closer to the Fermi energy than that of other clusters, which then enhances the electronic density of Pt. This facilitates the adsorption of HOCH₂CH₂O at the Pt sites and the activation of the C–H bond in HOCH₂CH₂O and therefore results in superior catalytic performance. This paper offers theoretical insights into the influence of Pt size on the selective oxidation of ethylene glycol to glycolic acid.</p>