In 1385 Ferrer Saiol, protonotary of Queen Eleanor of Sicily, translated into Catalan Palladius?s Opus Agriculturae (?The Work on Farming?). This Latin work was widely known throughout the whole of the Middle Ages, and therefore a translation of Opus Agriculturae was wanted in the fourteenth-century's Kingdom of Aragon. A difficult aspect of Opus Agriculturae is its technical vocabulary. Being aware of it, Ferrer Saiol referred in his translation to his use of numerous reference books to undertake the best possible version. These medieval glossaries and vocabularies belong in his translation, explicitly as well as implicitly. We identify here books that Ferrer Saiol used for this version, like Papias?s Vocabularium, Johannes Balbus?s Catholicon, Simon of Genoa?s Clavis Sanationis and Ibn Wafid?s De Simplicibus Medicinis. These books are analysed here in order to understand its translating process. In addition, we aim to understand through medieval glossaries those passages whose scientific terms Saiol doesn?t correctly translate.