Financial competence has become increasingly important for citizens, especially in decisions about complex financial products, such as mortgages. Mortgages have long-term consequences for individuals’ financial situations and potentially for the economy. To prepare for significant decisions, potential first-time homebuyers often consult the internet. Therefore, we analysed whether the information provided online by banks supports the development of financial competence about mortgages. We included webpages of two German banks differing in quality and quantity of information. The study comprised 101 students. They were randomly assigned to one of two experimental groups or a control group. All students answered self-assessment questions about financial knowledge and behaviour and took a knowledge post-test. Students in the experimental groups surveyed the webpages of the respective bank. They were requested to use the information to work on an authentic case that required a decision about taking a loan to buy a single-family house. The decision was to be justified. While doing so, some randomly selected students thought aloud. Results of the post-test did not show significant differences in knowledge between the groups. Further, the experimental groups did not differ in quality of arguments for or against a decision. The think-aloud data show that webpages stimulate statements differently. Also, the data reveal common misconceptions of students. Overall, webpages do not contribute to the development of satisfactory financial competence. The students’ arguments and think-aloud data reveal starting points for improvement of webpages or instructional support.