This study examines the effects of speech style, syllable stress, and proficiency level on the production of the second-language (L2) Spanish vowels /e/ and /o/. The study addresses traditional descriptions of L2 Spanish (e.g. Stockwell & Bowen, 1965), which claim that English-speaking learners, unlike native speakers (NSs), reduce vowels in unstressed syllables and diphthongize /e/ and /o/ in stressed syllables. Additionally, it adds to previous research by investigating how speech style affects L2 Spanish vowels, how these effects change by course level, and how they compare to NS style-shifting. Data was gathered from 55 adult learners of Spanish (SLs) at three course levels, as well as 10 NSs of Spanish using two elicitation tasks at different levels of formality. A total of 7,740 word-medial tokens of /e/ and /o/ was extracted, and vowels' F1, F2, duration, and diphthongization were measured using Praat. ANOVA tests were run to determine the main and interaction effects of participant group, elicitation task, and syllable stress on these four response variables for each vowel. Significant main effects as well as interaction effects were found for group, task, and stress on the F1 and F2 of /e/ and /o/, as well as interaction effects, providing evidence that the SLs and NSs centralized their unstressed vowels, that /e/ and /o/ became more raised and peripheral as course level increased, and that the SLs at all levels peripheralized their vowels in formal speech.