The synthesis, structure and magnetic properties are reported of two disklike mixed-valence cobalt clusters [(CoCo6II)-Co-III(thmp)(2)(acac)(6)(ada)(3)] (1) and [(Co2Co11II)-Co-III-(thmp)(4)(Me3CCOO)(4)(acac)(6)(OH)(4) (H2O)(4)](Me3CCOO)(2)center dot H2O (2). Heptanuclear complex 1 was prepared by solvothermal reaction of cobalt(II) acetylacetonate (Co-(acac)(2)), 1,1,1-tris(hydroxymethyl)-propane (H(3)thmp), and adamantane-1-carboxylic acid (Hada), whereas by substituting Hada with Me3CCO2H, tridecanuclear complex 2 was obtained with an unexpected [Co-2(III) Co-11(II)] core. The core structures of 1 and 2 are related to each other: that of 1 arranges as a centered hexagon of a central Co-III ion surrounded by a [Co-6(II)] hexagon, while that of 2 can be described as a larger oligomer based on two vertex-sharing [(CoCo6II)-Co-III] clusters. Variable-temperature direct-current magnetic susceptibility measurements demonstrated overall ferromagnetic coupling between the Co-II ions within both clusters. The magnetic exchange (J) and magnetic anisotropy (D) values were quantified with appropriate spin-Hamiltonian models and were also supported by density functional theory calculations. The presence of frequency-dependent out-of-phase (chi(M)'') alternating current susceptibility signals at temperatures below 3 K suggested that 2 might be a single-molecule magnet.