Millions engage in learning a Second Language (L2) using their mobile devices with a wide range of success. Concomitantly, there exists a growing interest in research on the effects of mobile-assisted language learning and predictors of learner outcomes (e.g., Loewen et al., 2020; Sudina & Plonsky, 2023). However, few of these apps and studies target L2 pronunciation, despite the association of mobile devices and oral communication. To this end, this dissertation study investigates the impact of Mobile-Assisted Pronunciation Training (MAPT) on English pronunciation skills through the lens of intelligibility and comprehensibility as well as its interplay with motivation and technology acceptance. A novel computationally driven instantaneous visual feedback system was developed along with an intelligibility/comprehensibility-centric 28-lesson course on high functional load vowel and consonant contrasts, lexical stress, rhythm, and features of discourse intonation. University-level L1 Spanish learners of English in Mexico (N = 122) were asked to complete the course over three weeks which included a pre-, post-, and delayed posttest, along with individual difference questionnaires at each test. Linear and logistic mixed effects models of listening tests and speaking tasks rated by trained listener participants (N = 30) indicated an improvement in intelligibility, comprehensibility, high functional load contrasts, and intonation signaling agreement. However, other suprasegmental features did not significantly improve. When interpreted with learner comments, these results suggest that stress and fluency features require more extensive and contextualized treatment. Taking a dynamic perspective on individual differences, the study also found that engagement in MAPT increased motivation, but extrinsic motivation is associated with less effort in MAPT. Social Influence to use MAPT became more important for finishing the course; but, along with Effort Expectancy, it was also associated with less participant effort. None of the individual difference subconstructs, however, were associated with learning outcomes as measured by an increase in comprehensibility scores. Taken together, these results support the use of mobile devices for enhancing L2 pronunciation for communication, confirm that a pedagogical approach that targets intelligibility and comprehensibility can be effective for L2 learners, and suggest that extrinsic motivation may hamper learner efforts in MAPT. Implications for theory, pedagogy, and technology are provided for educators, technology developers, and researchers interested in using mobile devices to promote spoken L2 communication skills. [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.]