This paper explores input factors that may cause variation in bilingual language proficiency in 38 bilingual children attending an international school. Results indicate a strong association between language exposure estimates and proficiency measures. The child's stronger language is shown to be a reliable predictor of variables related to language use. Abstract: The overall aim of this paper is to explore how certain linguistic factors relate to bilingual children's performance in English and French. The participants of our study are 38 French-English bilingual children aged six to eight, of middle to high socio-economic status, attending an international school in France. Data on the children's language experiences and family background were collected through two questionnaires -one completed by the parents, and the other given orally by the researcher to the children. Language proficiency was measured using the standardised French and English versions of the Peabody Picture Vocabulary Test, and the Student Oral Language Observation Matrix, a rating scale completed by the children's French and English teachers. Having examined the strength of the relationship between overall language exposure estimates and the language proficiency measures in each language, we then turn our attention to the relationship between each of the proficiency measures and, first, the amount of current language input to which the children are exposed, and second, the children's current amount of output in each language. Next we investigate the relationship between the child's stronger language and several variables related to language use, including the language the child finds easier to speak, the language the child prefers reading in and the language used with friends in the school playground.