Using high-resolution solid-state 15 N cross-polarization magic angle spinning NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 15 N-labeled porphycene were studied. Whereas at room temperature only a single 15 N resonance is observed, indicating an equivalence of all nitrogen atoms arising from a quasi-degenerate fast proton transfer, four signals are observed at low temperatures, exhibiting temperature-dependent line positions. Their analysis is consistent with the presence of either (i) two different molecules A and B in the asymmetric unit, each of which is subject to a quasi-degenerate correlated double proton transfer, or (ii) a single molecule exhibiting all four possible near-degenerate tautomeric states, two trans-and two cis-tautomers, interconverting by fast single proton transfers. The average rate constants of the proton transfer processes are found to be in the nanosecond time-scale. These constants were obtained between 228 and 355K by analysis of the longitudinal 9.12 MHz 15 N T 1 relaxation times, which exhibit a minimum around 280 K. The relaxation analysis was performed in terms of a quasi-degenerate two-state proton transfer process which modulates the heteronuclear 1 H-15 N dipole-dipole interaction. From the value of T 1 in the minimum, the crystallographic NN distance of 2.63 A Λand the hydrogen bond correlation for N-HΓΓΓN hydrogen bonded systems, the two NΓΓΓH distances of 1.10 and 1.60 A Λwere obtained, i.e. a hydrogen bond angle of 152Β°, which are significantly different from the corresponding values of 1.03 and 2.28 A Λand 116Β°found for porphyrin. The analysis of the temperature dependence of the rate constants indicates tunneling as a major reaction pathway, involving a barrier of about 32 kJ mol Γ1 . The finding of a larger NH distance and a smaller barrier for proton transfer as compared with porphyrin is rationalized in terms of the stronger intramolecular hydrogen bonds in porphycene. A strong coupling between these bonds would indicate that the proton tautomerism in porphycene corresponds to a correlated double proton transfer, in contrast to the stepwise transfer in porphyrin. Finally, a relation between the intrinsic 15 N chemical shifts of porphyrinoids and the NΓΓΓH distance was found, which might be useful for estimating geometries of porphyrinoids.