This study presents the results of an experiment designed to determine whether knowledge building can facilitate foreign language acquisition. We examined how groups of Catalan students in secondary schools worked together on collaborative writing tasks in English, their foreign language. The students were participating in the Knowledge Building International Project (KBIP), which is based on the concept that students can learn while working together in computer-assisted learning environments. The quantitative data was collected through a pretest and post-test. The pre- and post-tests were divided into four sections, which were grammar, vocabulary, long answer, and multiple-choice. The results show an increase overall in the performance of the foreign language, English. In particular, the analysis determined that the comprehension of the subject matter and writing abilities in the L2 showed an increase at high confidence levels, however, there are challenges in determining whether the students have acquired new vocabulary or developed a better understanding of grammatical rules, since it is difficult to predict which words and sentence structures they will use when participating in this project. [For the complete proceedings, see ED600837.]