Impedance spectroscopy (IS) and modulated differential scanning calorimetry (MDSC) were used to study the phase behaviour of NH4HSO4 (AHS) below room temperature. Phase transitions were clearly evidenced by dielectric measurements at about 270 and 154 K, but a large hysteresis of the low-temperature phase boundary was observed during thermal cycling of the samples. However, the thermal measurements at zero field (E = 0) only showed an endothermic peak at 154 K and thus appeared to be a first-order transition, but no thermal hysteresis was observed, which indicated a strong effect of the external field on the transition at this temperature. The phase transition at about 270 K, appeared to be a second-order one in which only dielectric measurements exhibited strong anomaly, but its intensity decreased remarkably on successive thermal cyclings of the sample. This behaviour is discussed in terms of disordering of the NH4+ orientations with thermal treatments of the sample that reduce the ferroelectric activity of the intermediate phase (154 K < T < 270 K) of AHS.