The input factors that may induce bilingual skill disparity were explored in 38 French–English bilinguals aged six to eight, of middle-to-high socio-economic rank, attending an international state school in France. Data on children's actual and cumulative language exposure and family background were gathered via surveys imparted to parents and children. Language proficiency was evaluated via the standardised French and English versions of the Peabody Picture Vocabulary Test and the Student Oral Language Observation Matrix, an oral skill rating scale, completed by children's French and English teachers. The outcomes stated notable links between language proficiency measures and actual amount of total exposure to each language, as well as actual input and output quantity. Cumulative exposure length also notably portended all the language proficiency measures. Moreover, the child's dominant language was a trustworthy indicator of variables linked to language use, including the child's preferential language for speaking and reading and the language employed with peers in the school playground. The implications of these outcomes are debated, pointing out especially the necessity to find ways of fostering the child's weaker language.