The 18 O isotope shifts become significantly larger at low pH, as shown in figure ). This effect has not been analyzed in detail, but is provisionally attributed to small changes of the equilibrium fractions of the different protonated species upon O substitution. O substitution changes the zero-point vibrational energy of -OH bonds and therefore influences the equilibrium fractions of the various protonated species, which leads in turn to a perturbation of the 13 C chemical shifts when averaged over all exchanging species. This mechanism has been identified as the source of relatively large and remote O-induced isotope shifts in the C spectra of some carboxylic acids . In contrast to the observed T S values, only a weak pHdependence of T 1 is observed. For example, at 9.4 T we observe T 1 values of approximately 30 s for all pH values apart from pH = 4.1, where T 1 decreases to ≈ 15 s. In particular, T 1 does not display any notable features either at the pKa values of squaric acid, nor at the inflection point pH ≃ 10 at which the observed T S values start to drastically increase.