The Mn 7 (HOXO 3 ) 4 (XO 4 ) 2 (X ؍ As, P) compounds have been synthesized by using hydrothermal conditions. The arsenate phase was obtained under autogeneous pressure at 1703C. However, more drastic conditions at both pressure and temperature were necessary in the attainment of the phosphate compound. The crystal structure of Mn 7 (HOAsO 3 ) 4 (AsO 4 ) 2 was solved using single-crystal data. The unit-cell parameters are a ؍ 6.810(3) A s , b ؍ 8.239(2) A s , c ؍ 10.011(4) A s , ؍ 104.31(2)3, ؍ 108.94(3)3, ؍ 101.25(2)3. Triclinic, P-1 with Z ؍ 1. The isostructural Mn 7 (HOPO 3 ) 4 (PO 4 ) 2 phase was characterized from X-ray powder di4raction techniques. The crystal structure of both compounds consists of zig-zag chains constructed by dimeric edge-sharing Mn 2 O 10 octahedra linked through the MnO 5 trigonal bipyramids. The three-dimensional framework is completed by the connection between isolated MnO 6 entities to the dimers octahedra and trigonal bipyramids. The existence of hydrogenarsenate and hydrogenphosphate anions has been con-5rmed by IR and Raman spectroscopies. Magnetic measurements indicate the existence of antiferromagnetic interactions in both compounds, which are slightly stronger in the arsenate phase. ^2002 Elsevier Science (USA)