In this case study we investigate how demand-driven documentation can be used in open source projects. The study relied on questions and answers (demand-driven documenta- tion) posted on Stack Overflow and studied two open source projects, Apache POI and Firefox for Android, in depth. We found that the demand driven documentation provided by the respec- tive community is in general of good quality and contributed within an acceptable amount of time. We conclude that demand driven documentation can be suitable as a primary information source if other resources accompany it. However, the suitability is closely related to the size and activity of the project’s community.