The structure of a new inorganic solid acid Rb 3 (HSeO 4 ) 2.5 (H 2 PO 4 ) 0.5 trirubidium hydrogenselenate dihydrogenphosphate: (RbHSeP) has been examined by X-ray single analysis performed at room temperature.This compound crystallizes in triclinic space group P1¯ with lattice parameters a=7.637Å; b=7.825Å; c=12.320Å; α=106.85°; β=106.49° and γ=91.10°. It has a unit cell volume 671.5Å 3 and two formula units per cell, giving a calculated density of 2.732gcm −3 . The structure was solved from 2458 independent reflections and refined with 166 yielded parameters, weighted residuals of R 1 =0.0558 and WR 2 =0.1384 based on F 2 and F values, respectively. The main feature of the structure of (RbHSeP) is the coexistence of three ordered and disordered anions on the one hand, and of rubidium cations Rb + in the same crystal on the other hand. Rubidium cations and hydrogen-selenate and phosphate anions are linked with ionic bands whereas hydrogen-selenate and phosphate anions are linked each other with hydrogen bands leading, thus, to a three-dimensional aspect structure.The infrared spectrum performed at room temperature in the frequency range 4000–500cm −1 confirms the presence of structural disorder in this material and the coexistence of two different anions (H 2 PO 4 − and HSeO 4 − ) in the same crystal structure. The thermogravimetry (TGA) analysis and the differential scanning calorimetric (DSC) show the presence of a structural phase transition of the title compounds at 374K.