As described below, our initial measurements of the singlet decay time constant T S for the C 2 pairs in O-enriched 1,2-13 C 2 squarate were disappointing. In aqueous solution and neutral pH we observed a singlet decay time constant of around T S ∼ 30 s in a field of 9.4 T, which is slightly shorter than the value of T 1 under the same conditions. Conventional mechanisms such as CSA-induced relaxation are insufficient to explain this low value of T S . Furthermore, the observed value of T S for O-enriched 1,2-13 C 2 squarate was found to be strongly dependent on pH, with a rather sharp transition from short T S values at low pH to long T S values at high pH, approaching 300 s in low magnetic field (see figure ).