This dissertation investigates the process of learning vocabulary in a second language in a classroom environment. Its objective is to understand the relationship between the classroom interaction and the vocabulary activities found in the textbook, as well as how these mediators affect the construction of lexical knowledge. To achieve this, two classes of basic English formed by 12 and 13 year-old students have been analysed. The data include the printed exercises of vocabulary used in class (textbook and other supporting material), field observation, video-tapes and transcription of interactions. Among other results, the analyses have indicated that students and teachers often use conversational strategies, like instructional conversation, scaffolding and individual contribution, to construct meaning. However, these strategies usually happen when there is a break in the activity suggested by the book. They have also shown that the textbook creates a semantic map for students and teachers, providing the vocabulary and opportunities to review it. Generally, the vocabulary activities in the textbook cover all the aspects and levels of knowledge of words, but the activities of generative use seem to be restricted to oral presentations and roleplays, which implies non-spontaneous use of the learned vocabulary.