On the basis of the crystal chemistry and electronic structure of gamma-NaxCoO2, we explored many cobalt-based oxide systems and encountered the compound "Ca3Co4O9". Through the detailed structure analyses, the cobalt oxide has been revealed to be a misfit-layered compound, which consists of two interpenetrating monoclinic subsystems. The first subsystem [CoO2] has a Cdl2-type triangular lattice while the second subsystem [Ca2CoO3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca2CoO3](p)CoO2, where p stands for the b-axis ratio, b(CoO2)/b(RS) -0.62. A typical polycrystalline sample of the compound exhibits reasonable thermoelectric (TE) performance with thermopower S = 130 muV K-1, resistivity = 15 mOmega cm and thermal conductivity kappa = 1 W m(-1) K-1 at 300 K. The resulting figure-of-merit Z (S-2/rhokappa) = 1.1 x 10(-4) K-1 is comparable to that of polycrystalline samples of the gamma-phase, indicating that the present misfit cobalt oxide is a potential candidate for a thermoelectric material. (C) 2004 Elsevier B.V. All rights reserved.