Following in the hylomorphic tradition of Aristotle, Thomas Aquinas holds that all material substances are composed of matter and form. Like Aristotle, Aquinas also recognizes two different types of forms that material substances can be said to possess: substantial forms and accidental forms. Of which form or forms, then, are material substances composed? This paper explores two competing models of Aquinas’s ontology of material substances, which diverge on precisely this issue. According to what the author refers to as the “Standard (...) Model,” Aquinas’s view is that a material substance is composed of prime matter and substantial form. According to the “Expanded Model,” Aquinas’s view is that a material substance is composed of prime matter, substantial form, and all of its accidental forms. After outlining the main claims of each of the two competing models and considering two arguments in favor of the Standard Model, the author offers two arguments in favor of the Expanded Model. He argues that, given the way in which Aquinas argues for God’s simplicity in question three of the Prima pars, and the way in which he consistently describes the difference between an essence and a suppositum, or individual substance, throughout his works, there is good reason to believe that Aquinas thinks that the accidental forms of a material substance are included among its metaphysical parts. (shrink)