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)(H(2)O)(2)(OH)(2)P(2)W(25)O(94)](16-) (Co(7)) crystallizes in the triclinic space group P (a = 12.3403(6) A, b = 22.5966(11) A, c = 23.2645(12) A, alpha = 68.7830(11) degrees, beta = 83.7981(12) degrees, gamma = 78.5423(13) degrees, V = 5922.4(5) A(3), Z = 2) and is formed by six CoO(6) octahedra from two Co(3) trimers sustained by Keggin trivacant fragments held together by the bridge [CoW(7)O(26)(OH)(2)], which contains one tetrahedral CoO(4) 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.