Two cobalt-containing analogues of the mineral alluaudite, NaCo3(XO4)(HXO4)2(X = P,As), have been synthesized by hydrothermal methods and characterized by single-crystal X-ray diffraction. Crystal data : NaCo3(PO4)(HPO4)2, monoclinic, C2/c, a = 11.865(2) angstrom, b = 12.137(2) angstrom, c = 6.512(1) angstrom, beta = 114.12(1)degrees, V = 855.9(2) angstrom3, Z = 4, R = 0.0373; NaCo3(AsO4)(HAsO4)2, as above except a = 12.054(1) angstrom, b = 12.378(1) angstrom, c = 6.780(2) angstrom, beta = 113.03(1)degrees, V = 931.0(3) angstrom3, R =0.0177. The two compounds are isostructural. The framework consists of paralle infinite chains of CoO6 octahedra sharing their skew edges, which are linked by XO4 and HXO4 groups to form a three-dimensional architecture with two types of channels running along the c-axis. Sodium ions are located in sites within one channel with the other channel being occupied by OH groups. The structure is compared with that of a natural alluaudite. Magnetic susceptibility measurements on a powder sample of NaCo3(PO4)(HPO4)2 were performed.