NMR relaxometry combined with high-resolution solid-state NMR techniques has been explored as a kinetic tool for the study of ultrafast proton transfers in solids. Rate constants of proton transfer are obtained in the milli- to nanosecond time scale by analysis of the longitudinal spin-lattice relaxation times T-1 of heteronuclei located in such a way that their dipolar interaction to the mobile protons is modulated by the transfer process. The T-1 measurements are facilitated by proton cross-polarization (CP), magic angle spinning (MAS), and proton decoupling during the detection period. In contrast to the study of static powders, the CPMAS method also provides the equilibrium constants of proton transfer necessary to obtain the rate constants from the T-1 values. Heteronuclear longitudinal relaxation in the presence of proton transfer is described in the theoretical section for the cases (i) static powders, (ii) powders rotating at the magic angle, and (iii) powders where longitudinal relaxation is isotropically averaged by magnetization transfer. In case i relaxation is multiexponential and difficult to evaluate. In case iii relaxation is truly exponential and characterized by a single longitudinal relaxation time T-1, related in a straightforward way to the dipolar interaction and the equilibrium and rate constants of proton transfer. This case is, however, difficult to realize experimentally, by contrast to the MAS case ii. As shown theoretically, in this relaxation is quasi-monoexponential and governed in very good approximation by the same T-1 values as in case iii.