While it is widely acknowledged that numerous factors can account for children's degree of bilingualism, the dynamic nature of the bilingual acquisition process makes it complex to analyse the roles played by different factors (Pearson 2007; De Houwer 2011; Unsworth 2014). As linguistic exposure changes over time, the roles of different factors evolve, resulting in language balance shifts. The data presented come from year one of a five-year longitudinal study which investigates the impact of linguistic exposure on a range of oral language performance variables. Participants are two groups of French-English bilingual children attending an international state school in France. Twenty children are aged 5-6 and 30 are aged 10-11 at the start of the study. Data on children's family background and current and cumulative language exposure are collected through parent questionnaires. A guided interview with each child provides further data on participants' language biographies (Simon 2014). Language proficiency is measured using standardised French and English receptive vocabulary tests (EVIP and BPVS). In addition, participants narrate the wordless picture book, "Frog, where are you?" (Meyer 1969) in each language. The narrations are recorded then transcribed and coded in the CHAT format of CHILDES. Task order is counterbalanced for language. Bilingual development is assessed regularly throughout the five-year study using the same research tools. Drawing on data from year one of the study, this paper explores the effect of linguistic exposure on participants' receptive vocabulary size and their narrative performance in each language. The current analyses focus on fluency (number of syllables/minute; number of meaningful syllables/minute), complexity (syntactic complexity; syntactic variety; mean segmental type-token ratio) and accuracy (error-free clauses; correct verb forms) (Yuan and Ellis 2003). The study aims to provide guidance to parents and teachers on ways of fostering oral language performance in children with one less strongly developed language.