In this quasi-experimental study, the authors examined the effects of an intervention designed to teach upper secondary school students to take source information, such as author expertise, into consideration when selecting, processing, and using textual resources to complete particular multiple-document literacy tasks. The intervention centered on a contrasting cases approach framed by authentic curriculum-based classroom activities and was implemented over six weeks by teachers who had participated in professional development seminars. The findings demonstrated that students who had participated in the sourcing intervention placed more value on source information when selecting texts, invested more time and effort in processing the texts they selected, and more frequently attributed textual ideas to their respective sources compared with students who had participated in typical classroom activities instead. These effects were observed on far transfer tasks in which students worked with multiple documents on different topics in different situational contexts for different purposes, and were sustained over a period of 5.5 weeks. The discussion highlights the uniqueness of the current intervention work and centers on the aspects of the sourcing intervention that likely promoted these broad, sustainable, and transferable sourcing skills in students. Attention is also directed to several possible lines of future research in this area.