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In this article, the author sets out to examine the traces of Platonism in three alchemic writings of the Middle Ages attributed to Roger Bacon, the medical doctor Arnaldus de Villanova and Thomas Aquinas. Medieval alchemy has long been viewed as the theater of ideas and notions of Aristotelian origin whose adaptation to the practice of alchemy, that is, to distillation and sublimation, led to the formation of philosophical theories explaining how the art of transmutation became possible. It appears to us, however, that not all of these ideas derived from Aristotle. Plato, considered by the authors of alchemy as an alchemist, and his medieval school, were equally drawn upon, and in our view their influence was felt in the elaboration of these alchemical theories of the Middle Ages. This study of the three works endeavors to show how the alchemists employed the philosophical tradition that was theirs, in blending diverse forms of thought, those of Aristotle, Hermes or Plato.