The stability of Th4+ to reduction in water is studied by DFT methods. The standard reduction potential (SRP) of homoleptic complexes including Th(H2O)(9)(4+)) Th(H2O)(10)(4+), Th(NO3)(4), Th(NO2)(6)(2-), Th(NO3)(6)(2-), Th(COT)(2), Th(acac)(4), ThCp4, ThF4, and ThCl4 have been investigated. The values vary-widely (from -3.50 V for Th(OH)(4). to -0.62 V for Th(NO3)(4) depending on whether the ligands are redox active (noninnocent) or not. Several additional topics of thorium chemistry are explored, including the hydrolysis mechanism of ThO2(H2O), n = 1, 2, 4, and the solution phase nonzero dipole moment of ThCp4. Dinudear complexes are also characterized, including Th2O4, Th2O2(OH)(4), Th2O2(H2O)(8), Th-2(OH)(8)(H2O)(4), and Th-2(OH)(2)(NO3)(6)(H2O)(4) and condensed thorium complexes as [Th-4(OH)(6)(H2O)(12) ](10+) and [Th-6(OH)(14)(H2O)(12)](10+). For the Th-2(OH)(2)(NO3)(6)(H2O)(4) Binuclear complex, the first SRP is -0.82 V and the second is 1.59 V. The first SRP corresponds to the reduction of the ligand NO3-, and the second SRP corresponds to dissociative electron transfer to the NO32- ligand. The calculated formation constant of Th(EDTA)(H2O)(4) is in reasonable agreement with experiment. The different stereochemistries of the bidentate ligands NO2-, NO3-, and acetylacetonate (acac) around the thorium center have very similar stabilities.