The synthesis, crystal structure, and physical characterization of two new radical salts formed by the organic donors bis(ethylenedithio)tetrathiafulvalene (ET) and bis(ethylenediseleno)tetrathiafulvalene (BETS) and the Keggin polyoxometalate (POM) [SMo12O40](n-) are reported. The salts isolated are ET8[SMo12O40]center dot 10H(2)O (1) (crystal data: (1) monoclinic, space group l2/m with a = 13.9300(10) angstrom, b = 43.467(3) angstrom, c = 13.9929(13) angstrom, beta = 107.979(6)degrees, V = 8058.9(11). angstrom(3), Z = 2) and BETS8[SMo12O40]center dot 10H(2)O (2) (crystal data: monoclinic, space group l2/m with a = 14.0878(2) angstrom, b = 44.1010(6) angstrom, c = 14.0930(2) angstrom, beta = 106.739(3)degrees, V = 8384.8 angstrom(3), Z = 2). Both compounds are isostructural and consist of alternating layers of the organic donors (with an alpha or theta(42+40) packing mode) and POM anions. The structural data, as well as the magnetic susceptibility, ESR measurements, and band structure calculations, indicate that the Keggin POMs have been reduced by one electron in 1 and by two electrons in 2, leading to the POMs [SMo12O40](3-) and [SMo12O40](4-) in 1 and 2, respectively. At ambient pressure 1 is a classical semiconductor with a room temperature conductivity of about 1 S cm(-1) and an activation energy of about 130 meV. Salt 2 also exhibits an activated behavior although it does not follow a classical semiconducting regime. The conductivity of 2 under applied pressure shows an enhanced conductivity, in agreement with the analysis of the electronic structure, although no metallic behavior is detected below about 10 kbar.