This study aims to test the Perceptual Uncertainty Hypothesis proposed by Daland et al. (2015). According to the hypothesis, variability in loanword adaptation reflects perceptual uncertainty; thus, one would expect to see speakers systematically refer to orthography only when phonetic similarity fails to identify a unique segment. We present an online experiment in which Korean and Japanese listeners heard two-syllable English nonce words like m？pple. They were asked to identify the closest vowel in their own language for English schwa in the second unstressed syllable. The results of the study provide strong support for the Perceptual Uncertainty Hypothesis since they show orthographic effects in the adaptation of unstressed vowels of English words loaned into Korean and Japanese. They demonstrate that the way that a word was spelled had some influence on which vowel the participants chose. The results also showed a very different pattern between Korean and Japanese. We can explain the results in terms of how the phonetic categories of English align with those of Korean versus Japanese.