The Vocabulario de la lengua zamuca is the only extant dictionary of Old Zamuco, an extinct Zamucoan language spoken in the 18th century in the abandoned mission of San Ignacio de Samucos, located in the northern Chaco lowlands of South America. This document was until now inaccessible to scholars, but has now been thoroughly studied by the present author, and found to contain very rich data, which establish its fundamental importance for linguistic studies on Zamucoan and Chaco languages. The critical edition of the dictionary is currently under publication. The original author of the dictionary, the Jesuit Father Ignace Chomé (1696-1768), reveals his brilliant linguistic intuition and remarkable skills in the collection and representation of linguistic data. One of the theoretical challenges he had to face was the threefold system of nominal suffixation, which is an absolute linguistic rarity. The present paper will show how the author dealt with the main word classes of Old Zamuco, that is verbs, nouns and adjectives. Chomé structured the entries for these lexemes in such a way as to provide plenty of information on inflectional and derivational morphology. The data from the dictionary permit new and interesting insights on the grammar of Old Zamuco.