Monoclinic RbD2PO4 polycrystals (DRDP) were synthesized via the room temperature crystallization of RbH2PO4 CROP) dissolved in D2O. Powder X-ray diffraction (XRD) data collected at T = 25 C indicate that this deuterated compound crystallizes in spacegroup P2(1)/m with unit cell parameters a = 7.688 A, b = 6.192 angstrom, c = 4.781 A and beta = 109.02, and is isomorphic with the intermediate-temperature phase of its hydrogenated counterpart RDP. We found no evidence of a previously reported [Phase Transitions 80, 17 (2007)] polymorphic phase transition in DRDP upon heating from room temperature to 210 degrees C. All lattice parameters vary smoothly within this temperature range, demonstrating that the P21/m phase persists upon heating. In addition, the unit cell volume of monoclinic DRDP is 1% greater than that of its RDP polymorph at all temperatures between 150 degrees C and 210 degrees C, which indicates the absence of significant deuterium-hydrogen isotope exchange. Further heating to 240 degrees C leads to the thermal decomposition of the title compound via dehydration. (C) 2013 Elsevier B.V. All rights reserved.