At all magnetic fields, the T S values of O-enriched 1,2-13 C 2 squaric acid vary strongly with the pH value of the solution. The overall pH dependence resembles a sigmoidal curve. For example, at 9.4 T the nuclear singlet decay time constant of O-enriched 1,2-13 C 2 squaric acid at pH = 1.3 is given by T S = 18.3 s. Initially, increasing the pH does not lead to any significant changes in T S . However at a pH value of ∼ 11 a sharp increase in T S is observed reaching values up to T S = 53 s. The maximum T S value of 275 s is reached for pH = 14.0. Similar behaviour is observed at 16.5 T and 2 T with an overall increase in relaxation times as the magnetic field strength decreases. For example, at 16.5 T, T S reaches a maximum of T S = 121 s, whereas at 2 T the observer singlet decay time constant reaches a maximum of T S = 380 s. The increase in T S with a decrease in the magnetic field strength is consistent with a dominant chemical shift anisotropy relaxation mechanism in this pH range.