The Phonological Permeability Hypothesis (PPH, Cabrelli Amaro & Rothman, 2010) attempts to reconcile evidence suggesting some L2 learners, however rare, attain native-like L2 phonological systems with the observation that most do not. Considering existing L2 phonology research, it is not clear that phonological differences between early and late acquirers must be the consequence of maturational effects on implicit mechanisms. Thus, to test how native-like adult-acquired systems are, the extent to which early- and late-acquired systems are equally resilient to influence from an L3 is examined. The PPH posits that native-like phonological systems acquired in adulthood are different from systems acquired in childhood with regards to relative stability. To test the PPH, a cross-sectional study of three types of English/Spanish bilinguals differing in age and context of acquisition (AoA) of Spanish was carried out to examine whether AoA determines relative vulnerability of the Spanish phonological system when exposed to L3 BP. In addition, a longitudinal case study was conducted, observing an L1 English/L2 Spanish bilingual's Spanish prior to BP exposure, and his BP and Spanish after 11 weeks of BP immersion. The focus of the investigation was the acquisition of reduced word-final unstressed vowels [I] and [near-close near-back rounded vowel] in BP and potential regressive influence on the perception, production, and processing of Spanish word-final unstressed [e] and [o]. Results from the cross-sectional study did not reveal any between-group differences in perception, production, or reaction time in terms of BP influence on Spanish. Therefore, these data align with the possibility that AoA does not determine the relative stability of a phonological system in terms of mental representation or processing. However, while these data do not support the PPH, evidence meeting the criteria for falsification of the PPH is also lacking. The case study results are indicative of rapid and permeating L3 influence on the learner's Spanish processing during speech production, and a large-scale longitudinal investigation of the three bilingual types tested here is necessary to reveal whether early and late acquirers of Spanish are equally vulnerable to L3 influence and what the nature of any observed vulnerability is. [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.]