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This paper tests the predictions of the vocabulary-tuning model of second language (L2) rephonologization in the domain of L2 segmental production. This model proposes a facilitating effect of adults’ L2 vocabulary expansion on L2 perception and production and suggests that early improvements in L2 segmental production may be positively associated with an expanding L2 vocabulary. The model was tested in a study of the L2 vowel intelligibility of adult Japanese learners of Australian English, who differed only in the size of their L2 vocabularies. The results support the predicted association between L2 vocabulary size and L2 vowel intelligibility and the prediction that early-phase L2 vocabulary expansion leads to improved L2 production.

Adult second-language (L2) learners’ perception of L2 phonetic segments is influenced by first-language phonological and phonetic properties. It was recently proposed that L2 vocabulary size in adult learners is related to changes in L2 perception (perceptual assimilation model), analogous to the emergence of first-language phonological function (i.e., attunement to the phonological identity of words) associated with the “vocabulary explosion” at 18 months. In a preliminary investigation of the relationship between L2 perception and vocabulary size, Japanese learners of Australian English identified Australian English vowels, provided goodness of fit ratings, and completed a vocabulary size questionnaire. We adopted a “whole-system” approach, allowing learners to apply all native vowel system possibilities to the full L2 vowel system. Learners with a larger L2 vocabulary were more consistent in their vowel assimilation patterns, compatible with the L2 perceptual assimilation model.