This dissertation investigates the acquisition of Spanish diphthongs by adult native speakers of English. The following research questions will be addressed: 1) How do adult native speakers of English pronounce sequences of two vowels in their L2 Spanish at different levels of acquisition? 2) Can OT learnability models, specifically the GLA, account for the pronunciation of L2 diphthongs? If so, what constraints do learners use and how do these constraints interact? If not, what other model(s) might offer an improved analysis of L2 diphthongs? Participants completed two production tasks, a Nonsense Word task and a Question & Answer task. The participants were divided by level of acquisition--Beginner, Intermediate, Advanced--and there was a Native Speaker Control group. After the data was collected, F2 values and duration of vowel sequence were measured and used to categorize the pronunciations as monophthongs, diphthongs, or hiatus. It was found that the use of diphthongization increased with level of acquisition in the data for the Question & Answer task. Data from the Nonsense Word task did not reveal the same pattern; instead, the level of diphthongization was more or less equal across all levels of acquisition and with the Native Speaker Control group. The OT account was able to explain most of the data in this study. The GLA proved successful in demonstrating how constraints interact in the pronunciation of L2 diphthongs. However, there were L2 pronunciations for which OT could not account. It is suggested that linguistic models based on lexical frequency may offer insight into how to account for these pronunciations. [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.]