This article aims at describing and explaining the effects of a period of Study Abroad spent in the target language country (SA) on foreign language development. Such effects are analysed in the short- and mid-term and in contrast with the impact of a period of formal instruction (FI) previously spent in the home country (AH). It is hypothesised that because each of the two contexts has differential patterns of input exposure, both quantitatively and qualitatively, their effect on the participants' communicative and motivational development will also be different. The learners' abilities to avail themselves of opportunities for contact with the target language that the SA context provides are also taken into account. For this purpose, a sample of Catalan/Spanish EFL undergraduates who spent a compulsory three-month sojourn in an English-speaking university was analysed longitudinally over a period of three years. Intensive testing of the learners' oral and written abilities was carried out, and both global and fine-grained measurements were applied to the data. Comparisons were also drawn between the sample of non-native English speaking learners (NNSs) and baseline data from a similar sample of native English-speaking (NSs) exchange students.