This paper explores the Medieval Latin documentation of the Catalan territory for information on the Catalan terms that come from the Latin noun merx, -cis. First, as Joan Coromines suspected, it confirms the existence of a Catalan noun merç 'merchandise', although in masculine; its first appearance is attested in 1083 and the word has two meanings, 'rights on markets' and 'taxes on merchandise'. Secondly, the first appearance of mercer, noun derived from merç, has been established in 1067. And finally, this corpus shows the creation, without any continuity, of the Latin noun mercerius from the Catalan mercer.