Abstract Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X‐ray diffraction (XRD) were used to study the phase behaviour of NH 4 HSO 4 (AHS) above room temperature. For powder AHS measured during successive thermal cycles, the MDSC curves show an endothermic peak at 147 °C on heating scans and an exothermic peak at about 115 °C on subsequent cooling runs. However, after the second thermal cycle, a second endothermic peak starts to appear at about 135 °C on heating runs whose intensity increases on subsequent cycles. The thermogravimetric measurements on powder AHS show weight loss above 135 °C. X‐ray powder diffraction data for AHS heated to 140 °C during 24 hours, show the diffraction pattern for (NH 4 ) 2 S 2 O 7 . We discuss the endothermic effect at 135 °C in terms of a partial dehydration setting in at reaction sites distributed on the surface of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)