The article presents the translation of three letters and two versions of the epitaph on the death of Duke Alexander of Mazovia, written by Aeneas Silvius Piccolomini on June 1-6, 1444, in Vienna. Two of these letters are addressed to Giovanni Peregallo, while the last one is directed to Juan of Segovia. Both recipients had strong ties to the Council of Basel. Aeneas himself was initially aligned with the Baselians following the schism of 1439; however, during the years 1442-1445, his sympathies shifted from pro-conciliarist to pro-papal. The letters examined in this article serve as evidence of this gradual transformation in Aeneas’ stance, influenced by various factors such as Emperor Frederick III’s opinion, conflicts within the conciliarist faction, and the strategic maneuvers of the future pope. Accompanying the translations is an extensive introduction providing background information on the events depicted in the letters and their key figures (Duke Alexander of Mazovia, Marcus Bonifili, and Laurentius of Rotella), analyzing Aeneas’ narrative through the lens of the “semantics of the turn” framework proposed by German scholar Johannes Helmrath.