Solid-state structure of the crystalline 1:1 complex [C₃H₁₀N(18-crown-6)]⁺[I₃⁻] (1) between 18-crown-6 and n-propylammonium triiodide has been determined at 293 and 93 K, respectively, showing a change from monoclinic P2₁/m to monoclinic P2₁/a. Crystal structural analysis shows that in addition to van der Waals’ forces, conventional N-H…O hydrogen bonds are the key interactions. Measurements of unit cell parameters versus temperature show that the values of one of the three axes and the crystal volume change abruptly and remarkably at 220 K, indicating a first-order phase transition. The lack of the mirror plane in the low temperature structure is the most important differences between the two structural forms. Differential scanning calorimetry (DSC) measurement confirms that 1 undergoes a reversible phase transition at about 220 K with a thermal hysteresis of 3.5 K. The relatively large latent heat makes 1 a good candidate for phase change materials. The phase transition is accompanied by an anomaly of dielectric constant during heating and cooling process near the phase transition temperature.