What axioms governing set-existence does Zermelo rely on in Zermelo (1908a)? At the start of the paper, Zermelo list two ‘postulates’ that he explicitly depends on, a version of the separation axiom, and the power set axiom. Later on he lists Axiom IV, which, as noted, asserts the existence of a choice set for any set of disjoint non-empty sets. In addition to this, Zermelo makes use of the existence of various elementary sets, though he doesn't say exactly which principles he relies on. In the axiomatisation which follows two weeks later, Zermelo adopts all these axioms, but adds clarification about the elementary sets. He also adds the Axiom of Infinity, to guarantee that there are infinite sets, and the Axiom of Extensionality, which codifies the assumption that sets are really determined by their members, and not by the accidental way in which these members are selected. In addition, as we have noted, he now calls the Axiom of Choice by this name. 