A transport reaction synthesis technique has been used to prepare single crystals of two pyroborate compounds having the formulas Cu2NiO(B2O5) and Cu2MgO(B2O5). The two compounds are isostructural and crystallize in the monoclinic space group P2(1)/c. Cu2NiO(B2O5): a = 3.2003(10), b = 14.775(3), c = 9.097(3), beta = 93.28(4), V = 429.4(2) Angstrom(3), Z = 4; and Cu2MgO(B2O5): a = 3.2401(6), b = 14.790(2), c = 9.147(2), beta = 94.88(2), V = 436.7(2) Angstrom(3), Z = 4. The structures of Cu2MgO(B2O5) and CuMgO(B2O5) were, respectively, refined from 804 and 1000 independent reflections to the final residuals R1 = 0.0366, wR2 = 0.0911 and R1 = 0.0231, wR2 = 0.0644. Both compounds exhibit a chevron-like structure built up of ribbons, made of edge-connected copper and nickel-oxygen polyhedra, running along the (100) direction. These ribbons are connected from one another via oxygen atoms and the cohesion of the three-dimensional network is ensured by [B2O5] entities. Cu in part occupies the position for Ni or Mg, so that the compounds actually are solid solution compounds. Ni or Mg atoms are octahedrally coordinated by oxygen, while the two pure Cu sites show [4] and [4 + 1] coordination, for Cu(l) and Cu(2), respectively. The ELNES B-K edge spectra for the two compounds support that the borate group present is [B2O5]. (C) 2003 Elsevier Ltd. All rights reserved.