1-Aminopyridinium iodide (API) undergoes a reversible first-order phase transition (PT) II -> I at 384 K. The single-crystal X-ray diffraction studies have been carried out at 110, 280 and 401 K. The compound crystallizes in the monoclinic space group P2(1)/c (phase II) and exhibits ferroelastic properties. High temperature paraelastic phase (I) is described by the trigonal symmetry (space group R (3) over barm) with the 6-site cations disorder. The mechanism of PT is governed by both cationic dynamics and shifts of the I anions. The conductivity that emerges when approaching the PT temperature is believed to be associated with the charge transfer from I anion to the 1-aminopyridinium cation. (C) 2012 Elsevier B.V. All rights reserved.