The article analyzes information about the death of Grand Duke Yaroslav Vsevolodovich described in the History of the Mongols in the context of reports from Russian sources in the second half of the 13th–15th centuries, as well as the epistle of Pope Innocent IV to Alexander Nevsky dated February 22, 1248. In historiography the version of the poisoning of the Grand Duke described in the work of John de Plano Carpini is traditionally trusted. At the same time researchers often synthesize information from different (often asynchronous) sources without paying attention to the analysis of its origin and semantic orientation. The relevance of the research is related to the reconstruction of the stages of memory formation about the events connected with Yaroslav’s death. The purpose of the work is to assess the reliability of information about the death of the Grand Duke reported in extant sources. The author comes to the following conclusions. First, the interpretation of the Grand Duke’s death established in historiography is based on very unreliable information provided by de Plano Carpini, reproduced in the Russian Galician–Volyn chronicle but not reflected in sources written at roughly the same time in Yaroslav’s homeland, the Suzdal land. Second, over time the death of the Grand Duke acquired details, usually of literary origin, indicating the desire of scribes to give Yaroslav the aura of a martyr who suffered from the «pagans» for Christians rather than to report real facts. Third, if we assume a real basis in the report of slander against the Grand Duke by a certain Fyodor Yarunovich, it is worth returning to the hypothesis of V.T. Pashuto. He believed that the names Fyodor Yarunovich and Temer (mentioned in the History of the Mongols and in the message of Pope Innocent IV) could refer to the same person. The papal bull referred to him as a witness to Yaroslav’s conversion to the Catholic faith. This information could be perceived as discrediting the «honor and dignity» of the Grand Duke and could have been sufficient grounds for an attempt to disavow it by accusing the boyar Fyodor of slandering Yaroslav Vsevolodovich.