Although bilingualism or multilingualism is the norm around the world, the bulk of linguistic research has focused on monolingual populations. Current theories of second language acquisition maintain that accurate second language sound perception is 1) correlated with speakers' production ability on an individual level and 2) dependent on observable similarities between the native and target language. This project tests these hypotheses with experimental and naturally collected language data. Examining these effects is the first step toward identifying areas of difficulty for learners of any given second language, which will contribute to development of educational methods tailored to any individual learner and language in question. The results of this project have the potential to advance both English as a Second Language initiatives and foreign language education methods in the U.S. Using methods drawn from phonological, phonetic, and psycholinguistic research, this project tests the above predictions with data collected from an understudied population of Welsh-Spanish bilinguals in Argentina. This population has variation in age and language learning backgrounds which makes it an ideal testing group, while the nature of the Welsh vowel system provides a constrained set of testable acquisition challenges for native Spanish speakers. A reading task will elicit productions of Welsh and Spanish vowels from the test group and control populations of Welsh speakers in Wales and Argentine Spanish monolinguals. Statistical analysis of control group speech will be used to predict which Welsh vowel contrasts are likely to challenge native speakers of Spanish. Results will be compared to 1) bilinguals' accuracy in vowel identification and discrimination and 2) the similarity of bilinguals' vowel productions to those of the control groups. Understanding the challenges of vowel contrast acquisition for this population will result in scalable predictions regarding the nature and consequences of second language acquisition for the field of bilingualism.