We show a simple variant on Eigen's familiar temperature-jump method to measure rate constants. The sample is prepared in a sealed NMR tube, which is heated and then abruptly cooled. The NMR spectrum is then taken repeatedly until equilibrium is reestablished at the new temperature. The sample can be used over and over again. We demonstrate the technique on the reversible addition of 9,10-dimethylantracene to C-60. The C-60 contains H-2, and this provides an NMR signal upfield from TMS, well away from the rest of the spectrum. We show that the equilibrium constant for H-2@C-60 is the same as that for He-3@C-60,