Organic–inorganic hybrids represent a new type of material showing promising properties. In this report, sequential dielectric transitions have been studied in an organic–inorganic hybrid halide, (N-2-AP)CuCl₅·2H₂O (N-2-AP = N-(2-ammoniumethyl)piperazinium) (1). The packing structure of 1 displays discrete [CuCl₅]³⁻ rectangular pyramids and N-2-AP cations, which are linked by two water molecules, forming infinite hydrogen bond networks with inorganic and organic components along the b-axis. Characterization studies containing differential scanning calorimetry (DSC) measurements, variable-temperature X-ray diffraction and dielectric measurements were performed to investigate the phase transitions in 1. The deuterated sample of 1 (named 2) also exhibits a similar behavior to that in 1, but shows different phase transition temperatures in dielectric transitions. The arresting deuterated effect strongly confirms that the phase transitions in 1 are attributable to the local vibrations of water molecules resulting from the variation of hydrogen-bonding interactions.