A novel molecular phase transition compound, tri-n-butylammonium trichloroacetate (TBAT), has been successfully synthesized with a reversible phase transition at 196 K. Its phase transition behaviour was confirmed by specific heat capacity and differential scanning calorimetry (DSC) measurements with a 2 K thermal hysteresis, indicating that the phase transition is a first-order one. Dielectric measurements further reveal the reversible nature of the phase transition, which exhibits distinctive step-like anomalies between low and high dielectric states. Moreover, temperature dependent single-crystal X-ray diffraction analyses of TBAT disclose that the order–disorder transformation of the flexible tri-n-butylammonium cations and the haloacetic acids anion together with the torsion in the cations stimulate the structural phase transition. All these results open a new way to design and assemble novel phase transition materials.