This study investigates vowel reduction in unstressed syllables in German Learner English, comparing learners at three different proficiency levels with native speakers of Standard Southern British English. Two acoustic properties of unstressed vowels were measured: duration and formant structure. Based on Major’s (2001) Ontogeny Phylogeny Model of second language acquisition, a U-shaped developmental pattern was predicted. The results follow the predicted pattern and seem to support the chronological corollary of Major’s model: Initial transfer of L1 stress-timing properties appears to facilitate vowel reduction. The subsequent influence of universal structures surfaces in an overarticulation of unstressed vowels. The final increase of target language structures leads to a higher degree of vowel reduction.