The synthesis and the structural and magnetic characterization of a novel heptanuclear cobalt cluster encapsulated in a heteropolyoxotungstate is reported. This complex shows how it is possible to control the nuclearity of the Co clusters formed in a tungstate solution by slightly changing the synthetic conditions, and the relevance of pH in this regard. This heptanuclear complex [Co-7(H2O)(2)(OH)(2)P2W25O94](16-) (Co-7) crystallizes in the triclinic space group P (1) over bar (a = 12.3403(6) Angstrom, b = 22.5966(11) Angstrom, c = 23.2645(12) Angstrom, alpha = 68.7830(11)degrees, beta = 83.7981(12)degrees, gamma = 78.5423(13)degrees degrees, V = 5922.4(5) Angstrom(3), Z = 2) and is formed by six CoO6 octahedra from two Co-3 trimers sustained by Keggin trivacant fragments held together by the bridge [CoW7O26(OH)(2)], which contains one tetrahedral CoO4 unit. The magnetic properties of the complex are discussed on the basis of the coexistence of ferro- and antiferromagnetic interactions and fitted according to an anisotropic exchange model in the low-temperature regime.