This study investigated the effects of pronunciation instruction for 36 tertiary learners of Korean in terms of accentedness, comprehensibility, and phonological error rates while accounting for proficiency (2nd-semester beginners, 4th-semester intermediates), first language background (English, Chinese), and task effects. Participants completed beginner-appropriate picture description and read-aloud tasks at pretest and posttest. Over eight weeks, a treatment group received eight hours of instruction targeting segmental and syllable-structure features. Ten Korean NSs judged learner accentedness and comprehensibility, and learner productions were analyzed for phonological errors. Beginners’ accentedness, comprehensibility, and error rates improved regardless of receiving instruction. For intermediates, the control group showed no development in comprehensibility or accentedness, while the treatment group trended toward increased comprehensibility. Intermediate students showed minimal improvements in phonological error rates. L1 had a negligible impact on accentedness and comprehensibility, but Chinese-speaking students committed more syllable-structure errors. Associations between phonological errors and comprehensibility/accentedness varied by outcome task and L1.