The characterization of the Aragonese used in the Heredia's works has met with surprise almost always on account of its heterogeneity, accountable by the different models used and the diverse people that have played a role in his scriptorium. To this should be added the different peculiarities of the language itself used by his patron, a language that has never been taken into account up to now. In order to fill this gap this paper presents an edition and a study of a letter from Castellan de Amposta, kept in the Archivo de la Corona de Aragon. This analysis reveals the use of a variety of linguistic norms: forms habitually found in Aragonese are to be found side by side in the letter with forms usual in Catalan. There are also forms that coincide with Castilian usage.