This article examines evidence provided by quantitative and qualitative measurement of second language (L2) proficiency by elementary-level learners of French, following participation in a school foreign exchange visit. Based on a study consisting of a controlled analysis of test results from 68 pupils studying French at three secondary schools in England, the paper discusses the short-term impact on performance in the four language skills as well as longer-term correlations with the pupils' results in the public GCSE examination. In addition, the paper attempts to define the profile of improvers in each language skill (as four separate subgroups) through analysis of their results in the four tests and through correlation with their reports on linguistic support provided by their exchange partner and host family. The findings suggest that the exchange had greatest impact on the strengthening of proficiency in listening comprehension and in writing performance (in relation to communicative content and fluency, but not with respect to grammatical accuracy). The paper also presents a more qualitative account of the writing test data, reporting on the output in terms of variety of tenses used and instances of expressive language. (Contains 10 tables.)