Far-infrared spectra and molar heat capacities of 5-iodo-9-hydroxyphenalenone (IHP) and its deuterated analog (IDP) were measured at temperatures between 5 K and ambient temperature. A strong absorption at 68 cm(-1) in IHP, for which the counterpart is missing in IDP, was assigned to the proton tunneling. A heat capacity anomaly due to a phase transition occurred at 36.0 K in IDP with Delta H-trs = 167 J mol(-1) and Delta S-trs = 5.45 J K-1 mol(-1). No phase transition occurred in IHP, proving mutual exclusion of the tunneling and order-disorder transition The results provide firm experimental evidence for discrete tunnel states in hydrogen-bonded crystals, the key concept in the theory of deuterium isotope effects in ferroelectricity.