Apart from leading to an understanding of singlet relaxation in systems undergoing chemical exchange, the results described here indicate that nuclear singlet relaxation may be an exquisitely sensitive reporter of rare chemical events. For example, an individual molecule of O-enriched 1,2-13 C 2 squarate, in a solution with pH ∼ 11, only picks up a proton about once a minute on average, and that proton is resident for only a fraction of a millisecond, on average, before returning to the solvent. The 13 C 2 singlet relaxation measurement detects the rare protonation events by means of a significant fall in the T S value. It is hard to find other methods for the reliable characterisation of such infrequent chemical events under equilibrium conditions.