The purpose of this study is to investigate the construction of authorial positioning in written argumentative texts produced by 12th grade students, regarding this discursive practice as a social semiotic space for creating strategies of argumentation and meaning negotiation. Considering writing as a conscious, planned and carefully thought activity, this research is theoretically supported by: the language perspective proposed by Systemic Functional-Linguistics (Halliday e Hassan, 1989; Halliday e Matthiessen, 1999; Eggins, 2004; Gouveia, 2009); the Appraisal System (Martin e White, 2005; Vian Jr, 2002); Genres Studies (Martin, 1992; Bakhtin, 1992; Miller, 1994) and theories of Argumentation (Breton, 1999; Abreu, 2003). Argumentative texts written by 12th grade students, which simulate compositions proposed in university entrance exams, were selected in order to investigate the construction of authorial positioning. Results indicate that Appraisal resources enhance the argumentative potential in the construction of authorial positioning, acting as possible strategies in the argumentation process. Considering the theoretical contribution, data analysis suggests a close relationship between language meaning potential and text argumentative potential, which are realized through semantic and lexical choices and are related to strategies of argumentation that result in argumentative meaning potentials. Therefore, according to this study, it is possible to create a parallel between the language perspective proposed by Systemic Functional-Linguistics and studies of argumentation. Implications of the study point to the latent necessity of using the Systemic-Functional and Appraisal frameworks in the classroom, in the teaching-learning of Portuguese Language, mainly in relation to the Written Production.