The crystal structure of a crystalline artefact of a reaction involving copper(I), cyanide and water under conditions of high temperature and pressure has been determined by single-crystal X-ray diffraction methods at 295 K, and refined by full matrix least-squares to a residual of 0.046 for 572 independent 'observed' reflections; the composition is suggested to be [Cu3(CN)3(H2O)]∞, comprising a two- dimensional polymer with both linearly coordinated and bridging cyanide groups. Crystals are monoclinic, probable space group C2/c, a 19.048(7), b 6.798(3), c 13.223(7) Ǻ, β 125.72(3)°, Z 8.