Reading comprehension plays a fundamental role in the processes of acquisition, sharing, and construction of knowledge. Many students experience problems when they are engaged in reading comprehension of expository texts. The interdependencies between reader's characteristics and text properties contribute especially to reading comprehension performances. The aim of this study is to explore the direct and indirect effects of prior knowledge on the comprehension of two texts with different disciplinary content. A battery of tests has been administered to 131 7-graders. Data have been explored through analysis of variance and regression analysis. Results confirm the centrality and the multidimensional nature of reader's prior knowledge. The effect of his component is both, direct and mediated by inferences, but with different modalities depending on the text's disciplinary content.