Using dynamic liquid-state H-1 NMR and solid-state N-15 CPMAS NMR spectroscopy (CP=cross polarization, MAS=magic angle spinning), the intramolecular double-proton transfer (tautomerism) of N-15-labeled porphyrin has been studied. Rate constants including kinetic HH/HD/DD isotope effects were obtained as a function of temperature not only for the liquid phase but also for the polycrystalline solid. Within the margin of error, the liquid-state degeneracy of the tautomerism is maintained in the solid state; moreover, rate constants and kinetic isotope effects are the same for both environments. Therefore, it is justified to combine rate constants of the porphyrin tautomerism determined for various environments into a single Arrhenius diagram. The Arrhenius curves of the different isotopic reactions indicate a stepwise tautomerism via a metastable intermediate as postulated theoretically, involving thermally activated tunneling at low temperatures. Furthermore, the observed kinetic isotope effects do not support a solid-state process consisting of combined nondegenerate proton transfers and molecular 90 degrees rotations. This possibility has been previously discussed in order to reconcile the observations of proton disorder in solid porphyrin by NMR and proton order by X-ray crystallography. Finally, a novel process has been observed which consists of a complete reversible deuteration/ reprotonation of the mobile proton sites of solid porphyrin by contact with gaseous or liquid D2O/H2O. The process involves (i) penetration of gaseous water into the porphyrin crystals, (ii) diffusion processes of water or of porphyrin inside the crystals, and (iii) exchange of protons or deuterons in molecular hydrogen-bonded water-porphyrin complexes.