The present article provides an exploration of ultimate attainment in second language (L2) and its limitations. It is argued that the question of maturational constraints can best be investigated when the reference population is bilingual and exposed on a regular basis to varieties of their first language (L1) that show cross-linguistic influence. To this end, 20 advanced Dutch-English bilinguals are compared to 9 English native speakers immersed in a Dutch environment. All participants are teachers or students of English at a Dutch institution of higher education. The populations are shown to be at similar global proficiency levels. Two phonetic variables (voice onset time or VOT and vowel discrimination) and one grammatical variable (verb phrase ellipsis), which are assumed to present particular challenges to Dutch learners of English, are explored, and speakers are furthermore rated for their global nativeness. The findings show no differences between populations on VOT but some variance on the production of a vowel that has no correlate in Dutch (the English trap vowel). However, all but one of the L2ers are rated outside the range of the natives on perceived foreign accent. There are also differences between groups where acceptance of different sentence types with verb phrase ellipsis are concerned. We interpret these findings to indicate that there are areas of L2 knowledge and production that are persistently difficult to acquire even under circumstances that are highly favourable for L2 acquisition.