The high pressure synthesis, structure and magnetic properties of Ca25Co22O56(OH)28 are reported. The compound has a misfit structure, consisting of double, square calcium oxide hydroxide rock-salt-like layers between hexagonal CoO2 layers. The misfit compound crystallizes in the monoclinic space group C2/m, and can be characterized by the coexistence of two subsystems with common a = 4.893(5) angstrom, c = 8.825(9) angstrom and beta = 95.745(8) parameters, and different b parameters: b(RS) = 4.894(5) angstrom, and b(HEX) = 2.809(3) angstrom, for the rock-salt and hexagonal type planes, respectively. The compound shows Curie-Weiss paramagnetism with an antiferromagnetic Weiss temperature of -43 K and a reduced Co moment. Substantial deviations from Curie-Weiss behavior are seen below 50 K with no indication of magnetic ordering. No superconductivity was observed down to a temperature of 2 K. (c) 2006 Elsevier Ltd. All rights reserved.