Abstract This study explores the extent to which perceptual and other factors (i.e., orthography and standardization) affect vowel adaptation in English loanwords in Korean. It also examines whether perceptual variability gives rise to adaptation variability, and it further addresses whether the degree of perceptual influence in vowel adaptation is different from that in consonant adaptation. Data on Korean listeners’ perceptual mapping patterns for English vowels were collected in an English-to-Korean vowel category mapping experiment and compared to the patterns in a loanword database. The results indicate that perceptual factors have a great impact on the adaptation of English vowels into Korean, in line with de Jong and Cho’s (de Jong, Kenneth & Mi-Hui Cho. 2012. Loanword phonology and perceptual mapping: Comparing two corpora of Korean contact with English. Language 88. 341–368) findings for the adaptation of English consonants into Korean. Consequently, factors such as orthography and standardization are viewed as deviations from these perceptual influences. The impact of perceptual adaptation is, however, rather limited because the correlation between perceptual mappings and loanword mappings is less robust for English vowel adaptation than for English consonant adaptation possibly due to the less categorical nature of vowels relative to consonants. Additionally, the results show that increased strength of modal perceptual mappings is related to less loanword variability in vowel adaptation.