This work presents evidence (both historical and orthographical analysis) that demonstrates that a charter granting ownership of land around St. Nicholas Mukachevo Monastery, allegedly given by Prince Theodore (Fedor) Koriatovych, was falsified under Hungarian Catholic rule. It is argued that in particular, the copy published by I. Basilovits is a literary forgery emerging from at the earliest, the mid-15th century, after the death of Koriatovych, as the letter contains some orthographical features typical of this time, and could not have been written during the prince’s lifetime.