In two experiments, we examined fourth and fifth graders' comprehension of the source of information in texts presenting controversial issues. In Experiment 1, participants read short texts in which two people presented different arguments regarding an issue. Participants identified who said what and evaluated each source's knowledge of the issue. Most students correctly identified the source of information but failed to take into account each source's characteristics when determining who was most knowledgeable. In Experiment 2, we implemented an intervention on students' evaluations of information sources. Less skilled students in the intervention group assessed source knowledge more accurately than those in a control group. We conclude that elementary school students' comprehension of who says what in a text can benefit from training sessions that involve practice, feedback, explanations, and discussions. We consider the implications for document-based learning in elementary and secondary education. (Contains 5 tables.)