The World Wide Web is rapidly growing as a source of scientific information for the layperson. We analyzed Web presentations of scientific arguments and how students evaluate those arguments. Our findings suggest that argumentation can be described as a situated activity. Web sites presented challenges relating to multiple layers of argument, missing evidence and evidence that cannot be corroborated, and insufficient detail. These characteristics of Web sites exacerbated weaknesses in students' skills of argument. Student weaknesses stemmed from their epistemological stance, their overreliance on surface features rather than systematic analysis, metacognitive failures, and a failure to understand the nature of science and publishing.