Chapter 9 of Aristotle's "On Interpretation" deals with determinism, and here the two influential commentaries of Ammonius and Boethius have been published together. These are crucial works, for Ammonuis' commentary influenced the Islamic Middle Ages, while that of Boethius was of equal importance to medieval Latin-speaking philosophers. It was once argued that Boethius was influenced by Ammonius, and this work aims to show that this was clearly not the case. Ammonius draws on the fourth- and fifth-century Neoplatonists Iamblichus, Syrianus and Proclus. He arranges his argument around three major deterministic arguments and is our main source for one of them, the Reaper argument. Boethius, on the other hand, draws on controversies from 300 years earlier between Stoics and Aristotelians as recorded by Alexander of Aphrodisias and Porphyry.