The present article reports on the findings of a cross-sectional acoustic study of the production of the Spanish /e/-/ei/ contrast, as in "pena-peina" and "reno-reino," by native-English intermediate and advanced learners of Spanish. The acoustic parameter that distinguishes Spanish /e/ from /ei/ is formant change--/e/ is a monophthong and /ei/ is a diphthong. English, on the other hand, has a mid front monophthong /?/ ("bet," "debt") and a mid-to-high front diphthong /e[superscript ?]/ ("bait," "date"). These vowels differ in both their overall place of articulation and in their dynamic characteristics. Spanish /e/ is similar to both English /?/ and /e[superscript ?]/ but is not identical to any of the two vowels; Spanish /ei/ is similar, but also not identical, to English /e[superscript ?]/. This situation creates complex inter-lingual interactions for the learners. The study addresses these interactions as they are evidenced in speech production. The findings are discussed in terms of their significance for current models of second language speech learning and provide important information regarding the paths of pronunciation development in the second-language acquisition of Spanish vowels by adult native speakers of American English.