In two experiments, undergraduates' evaluation and use of multiple Internet sources during a science inquiry task were examined. In Experiment 1, undergraduates had the task of explaining what caused the eruption of Mt. St. Helens using the results of an Internet search. Multiple regression analyses indicated that source evaluation significantly predicted learning outcomes, with more successful learners better able to discriminate scientifically reliable from unreliable information. In Experiment 2, an instructional unit (SEEK) taught undergraduates how to evaluate the reliability of information sources. Undergraduates who used SEEK while working on an inquiry task about the Atkins low-carbohydrate diet displayed greater differentiation in their reliability judgments of information sources than a comparison group. Both groups then participated in the Mt. St. Helens task. Undergraduates in the SEEK conditions demonstrated better learning from the volcano task. The current studies indicate that the evaluation of information sources is critical to successful learning from Internet-based inquiry and amenable to improvement through instruction. (Contains 2 figures and 7 tables.)