Writing is a task that entails high cognitive and linguistic efforts, especially when producing academic texts. Academic language might be one of the factors influencing the quality of written texts, given that prior research has shown its impact on reading comprehension. The purpose of this study is to examine the contribution of Spanish Core Academic Language Skills (S-CALS) and academic vocabulary to the quality of written argumentation and explanation. For this study, 126 Chilean 8th grade students produced an argumentative text and an explanatory text about the same topic. In addition, their academic vocabulary was assessed with the S-AVoc-T test and their CALS with the S-CALS-I test. Results show that both CALS and academic vocabulary are significantly and positively correlated with both writing tasks. Even though these instruments make different contributions to the predictive models in each discursive genre, a Principal Component Analysis revealed that the model that best explains writing quality are those which combine both language variables, namely Spanish Core Academic Language and Vocabulary Skills (S-CALVS). In argumentation, the S-CALVS model explains 29% of the variance, after controlling by gender. In contrast, in explanation, S-CALVS explains 35% of the variance. It is concluded that it is relevant to develop situated writing in each discursive genre and, upon that basis, to work with both CALS and academic vocabulary, because they have a specific impact on academic texts writing.