We provide approximate theoretical expressions for the singlet relaxation rate constant of a spin-1/2 pair ensemble, using a simplified model of 2-site exchange. Three terms are shown to contribute to the singlet relaxation rate constant T -1 S in the presence of chemical exchange. The first two terms are the population-weighted averages of the T -1 S values for the participating species. These terms correspond closely to the theory developed for ligand binding applications of singlet NMR . Since these terms depend on equilibrium populations, they do not depend on the individual values of the kinetic rate constants, but only on their ratio. The third term is the most important one, in the current context. It depends on the individual kinetic rate constants of the chemical transformations and cannot be described as a populationweighted average of some quantity. We therefore call the third term the kinetic contribution to the singlet relaxation rate constant. Since proton exchange is usually a very fast process, the third term (kinetic term) can easily dominate, even when some of the participating species have negligible equilibrium concentrations. That is the key to understanding the remarkable pH-dependent T S data in figure .