X-ray diffraction, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electrical conductivity measurements were used to study the phase behaviour of NH4H2AsO4 (ADA) above room temperature. Our results indicate that dehydration occurs in the temperature range below the earlier reported onset of decomposition at 438 K; on heating two events are observed, at T-p1 = 361 K and at T-p2 = 423 K and hydrolysis takes place on cooling to room temperature under appropriate water vapour pressure. We suggest, in accordance with recent findings for similar compounds, that partial polymerisation sets in at reaction sites distributed on the surface of the crystals.