Japanese shows an asymmetry in the treatment of word-final [n] in loanwords from English and French: while it is adapted as a moraic nasal consonant in loanwords from English, it is adapted with a following epenthetic vowel in loanwords from French. We provide experimental evidence that this asymmetry is due to phonetic differences in the realisation of word-final [n] in English and French, and, consequently, to the way in which English and French word-final [n] are perceived by native speakers of Japanese. Specifically, French but not English word-final [n] has a strong vocalic release that Japanese listeners perceive as their native vowel [ɯ]. We propose a psycholinguistic model in which most loanword adaptations originate in perceptual assimilation, a process which takes place during perception and which maps non-native sounds and sound structures onto the phonetically closest native ones. We compare our model to alternatives couched within phonological theory.