Computational analysis of the arylation of aliphatic Csp3–H bonds catalyzed by decatungstate–nickel validates the reaction mechanism and provides valuable insights into how to accomplish the corresponding alkylation. Our analysis indicates that the light-excited decatungstate photocatalyst activates one of the Csp3–H bonds by a hydrogen atom transfer reaction involving the bridging O atoms of decatungstate. The generated alkyl radical reacts with a bipyridine-based Ni catalytic species to form a NiI-alkyl intermediate. Oxidative addition of aryl bromide to this intermediate leads to a NiIII complex, from which the cross-coupling product is liberated via reductive elimination. We also investigated the corresponding prototype reaction where the aryl bromide is replaced by an alkyl bromide, which suggested a too high energy barrier for the oxidative addition of the alkyl bromide to the NiI-alkyl intermediate. Variation of the steric and electronic properties of the bipyridine ligand suggests that steric encumbrance in the 6,6′ position can promote oxidative addition of the alkyl bromide as verified by ad hoc experimental tests.