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The purpose of this paper is to introduce the discourse analysis method called semantic profiling, which is a part of the Legitimation Code Theory (LCT). The main concern of LCT is the study of knowledge-building through discourse practices, particularly those performed in educational contexts. After introducing LCT, its goals, structure and relevant claims, a method to build semantic profiles is presented and explained in this work. This method is illustrated by the analysis of messages taken from an online discussion forum, where pre-service teachers of English collaboratively built knowledge about psycholinguistic concepts. Finally, implications of this analysis for educational practices are discussed.