The most common diagnostic tool to check for the evidence of tunneling in chemical reactions has been the measurement of kinetic isotope effects (KIEs). For a thermal reaction, replacing hydrogen in the broken or formed C-H bond with deuterium typically leads to a decrease in the reaction rates with KIE (kH/kD) of ~ 7 at 298.15 K. For KIEs beyond this semiclassical limit, it is generally realized that the reaction is driven by tunneling. We therefore studied the KIE, to test the tunneling effect, by taking 1H,H and 2H,OH as a model compound. Since the bond-breaking and bond-formation during the tautomerization involve significant movement of three hydrogens (H1, H2 and H3), as reflected in the eigenvectors corresponding to the TS imaginary frequency (see Fig. ), we checked the KIE(kH1H2H3/kD1D2D3) and predicted a dramatically large tunnelingcorrected KIET of 500.1 for 1H,H and 465.7 for 2H,OH at 298.15 K, giving a strong indication of proton tunneling in this systems. We further studied the KIEs of these hydrogens separately to dissect their individual contributions and find the "tunneling-determining atom" i.e., the atom that has the greatest influence on the tunneling rates. Figure shows the labeled atoms with their calculated KIEs with and without tunneling corrections.