The study of C nuclear singlet relaxation in Oenriched 1,2-13 C 2 squarate reveals a surprising dependence of the singlet relaxation time constant T S on the pH of the solution. A rather sharp transition is observed, from a short T S at pH values below ∼ 10, to long T S values above pH ∼ 12. The position of this transition is more than 6 pH points above the known pK a values of squaric acid.