This inquiry analyzed the relationships between individual differences and gains made in oral proficiency of adult, second language learners of Spanish during one semester studying abroad. Oral proficiency was measured using a pre/post-SA Computerized Oral Proficiency Instrument (COPI, CAL, 2009). Gain scores were correlated with two cognitive aptitude measures: 1) the Modern Language Aptitude Test (MLAT); 2) a phonological working memory test (WM); and 3) a series of motivational and affective aptitude measures including Willingness-to-communicate (WTC, McIntyre, 1992), motivation and other affective variables (Gardner, 1985; Yashima, T., Zenuk-Nishide, L., & Shimizu, K., 2004). The researcher concludes that the students made significant gains in oral proficiency during the experience abroad, both in terms of COPI scores and fluency (words/minute). Using the Language Contact Profile (LCP, Collentine & Freed, 2004) students reported speaking, listening and reading significantly more in Spanish than in English during the study abroad; however, they reported writing almost as much in English as in Spanish. There was a significant negative correlation between the MLAT and COPI gains; indicating that students who had higher language learning aptitude made fewer gains in oral proficiency while abroad. WM, WTC and other affective aptitudes were not correlated with COPI gains in this study; WM and L2 anxiety were significantly correlated with the pre-SA COPI. Finally, WM and the MLAT were significantly correlated, but no other aptitude measures collected were found to correlate. In the discussion the researcher reviews the inverse pyramid schema of the ACTFL guidelines (1999) and demonstrates the impact of the imprecise delineation between the levels. The contraction of the scale at the upper reaches leads to a ceiling effect for second language learners and does not allow an accurate depiction of growth in language skill and development. As the ACTFL scale is the standard in oral proficiency measurement in the United States, data collection instruments based on this scale do not meet the needs of researchers in SLA or educational settings that wish to investigate achievement in oral proficiency in varying context of language leaning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]