Factors which impact bilingual language development can often interact with different language features. The current study teases apart the impact of internal and external factors (chronological age, length of exposure, L2 richness, L2 use at home, maternal education and maternal L2 proficiency) across linguistic domains and features (vocabulary, morphology and syntax). Participants were 40 Arabic-speaking sequential bilinguals acquiring English (5;7-12;2, M = 8;4). Length of exposure predicted vocabulary and morphology, while chronological age predicted syntax. L2 richness also predicted vocabulary and syntax, although the impact on syntax was selective across structures. This split between syntax on the one hand, and vocabulary and morphology on the other, reflects the more embedded properties of the former; this contrasts with vocabulary and morphology, where transfer from the L1 and L2 may be more strongly dependent on the availability of shared forms across languages. Further implications are considered for sequential bilinguals in education contexts.