This study investigated upper secondary school students' skills in evaluating the credibility and argumentative content of a blog text and a YouTube video. Both sources concerned child vaccination, the blog text opposing and the YouTube video supporting it. Students rated each source as credible, fairly credible or non-credible, justified their ratings, and analyzed the argumentation of both sources. Their justifications were analyzed for trustworthiness and expertise and their argument analyses for identification of the main position of the source and the reasons supporting it. Students' justification skills proved fairly weak, and they also struggled with recognizing unbalanced argumentation. Students' skill in analyzing the argumentation used in the sources also proved inadequate, especially in the blog text task. Overall academic achievement significantly predicted students' credibility evaluation and argument analysis skills. The results suggest that greater emphasis should be placed on tasks involving the interpretation and analysis of online information.