The high-temperature behaviour of RbH2AsO4 (RDA) was studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The phase-transition like phenomena (i.e. DSC endothermic peak accompanied by a change of crystal symmetry from a tetragonal to monoclinic structure), named as a high-temperature phase transition (HTPT), have been observed in RDA around T-p = 150 degreesC. Our results show that this temperature marks the onset of a thermal dehydration of RDA similar to that suggested by Lee for the KDP family compounds on heating above T-p.