This study examined how Korean EFL learners in different proficiency levels attest contrasts of L2 phonology. To this end, twenty Korean EFL learners in high and low levels of English proficiency produced words with word-final contrasts of /[esh]/ vs. /[esh]i/, /t[esh]/ vs. /t[esh]i/, and /d[ezh]/ vs. /d[ezh]i/. Acoustic analyses of lexical vs. epenthetic vowels and lexically vs. derivationally final sibilants were conducted. The results of the acoustic analyses revealed that Korean learners differentiated them at the acoustic level by employing different vowel ratios or F1 for the produced forms with a final lexical or epenthetic vowel which did not differ at the segmental level. On the other hand, the low group did not acoustically differentiate the two types of vowels after /d[ezh]/ at all. With the production of a final sibilant either lexical or derivational, Korean learners also made distinctions in terms of different frication ratios. The results of the study found L2 learners distinguish segmental contrasts acoustically, which is different from L1 speakers, and the proficiency level partially mediates this. The results implicate the existence of interlanguage phonology with its independent system.