Most students profit from the easy access to online information, but specific competences for successful reading on the internet are seldom taught during class. Therefore, students might not be able to choose credible information autonomously. Empirical evidence suggests that high school students hardly evaluate the credibility of sources (“sourcing”) when reading from multiple documents. Consequently, effective interventions fostering sourcing skills are needed. This study evaluates the effect of a written instruction designed to augment sourcing activities in a multiple document reading task by inducing epistemic vigilance. The written instruction introduces the concept of the division of cognitive labor and informs about low editorial control on the internet. In comparison to a control group, students receiving the instruction prior to completing an internet research task showed more attention to, evaluation of, and memory for sources.