The synthesis of 1,2-13 C 2 squaric acid (3,4dihydroxycyclobut-3-ene-1,2-dione-1,2-13 C 2 ) will be described in a following publication. For all experiments 5.8 mg of 1,2-13 C 2 squaric acid[ 16 O 4 ] were dissolved in 0.5 mL of a 1:1 mixture of H 2 O:D 2 O leading to a concentration of 0.1 M. The isotopic purity of the H 2 O was 97.1%. Solutions were incubated at 80 ○ C for two hours allowing the O/ O exchange process to reach its dynamical equilibrium. The pH value of each sample was adjusted by step-wise addition of aqueous solutions of 0.2 M NaOH or 0.2 M HCl. In each instance the total volume added did not exceed 50 µL leading to a negligible effect on the sample concentration. The pH values were estimated by using a pH meter equipped with a Hamilton SpinTrode probe (www.hamiltoncompany.com), suitable for pH measurements inside NMR tubes. For the sake of simplicity, we ignore the potential complications associated with solvent deuteration, such as: (i) the influence of solvent deuteration on the acid-base equilibrium; (ii) the distinction between the concentration of the hydronium ion, indicated by pH, and that of its deuterated analogues; (iii) the modified response of the pH-meter, induced by solvent deuteration.