The present study investigates the acoustics (F1 × F2) of Catalan and Spanish mid-back vowels as produced by highly proficient, early Spanish-Catalan bilinguals residing on the island of Majorca, a bilingual speech community. Majorcan Catalan has two phonemic mid-back vowels in stressed positions (/o/ and /c/) while Spanish possesses only one (/o/). Two groups of bilinguals were recruited and asked to produce materials in both languages – one group of Spanishdominant and one of Catalan-dominant speakers. It was first found that Catalan and Spanish /o/ are virtually indistinguishable. Catalan /c/ is lower and more fronted than the other two vowels. Spanish-dominant bilinguals were found to differ from Catalan-dominant ones in that they did not produce the Catalan-specific /o/-/c/ contrast in their speech; that is, they produced a single, merged Catalan mid-back vowel. A within-subjects analysis of first- and second-language mid-back vowels further suggested, for Spanish-dominant bilinguals, that they had developed a separate vowel category to accommodate their single, merged Catalan mid-back vowel; that is, they possessed a two-category mid-back vowel system, i.e. one for their Spanish /o/ and one for their merged Catalan /o/ + /c/. Potential explanations and theoretical implications are discussed.