This paper sheds new light on two Greek texts accompanying Aeschylus’ Prometheus Vinctus, in the fifteenth-century manuscript Q No. 2 of the Library of the Russian Academy of Sciences in Saint Petersburg. The first text is a didactic poem on iambic versification, allegedly composed by Michael Psellos, and the other one is a mixture of book epigrams related to the subject of the Prometheus Vinctus. August Nauck studied the manuscript and published these texts. All further mentions of the manuscript depend on Nauck’s readings, which nobody seems to question. In the latest edition of Psellos, prepared by Westerink, the manuscript from St Petersburg has not been taken into account, albeit the editor mentions Nauck’s publication. As for the epigrams, they have been published several times, also without taking that manuscript into account. A new study of the codex shows that Nauck’s edition contains several minor misreadings, therefore, I propose a new edition, based on the St Petersburg manuscript, as well as other manuscripts bearing same or similar verses, which were, apparently, unknown to him. Analyzing the epigrams on Prometheus, I compare our manuscript with others which contain the same verses (usually in different order). I try to explain some of the mistakes in these texts and correct them, as well as to compare them with other readings.