The use of viewpoints has long been proposed as a technique to structure evolving requirements models. In theory, viewpoints should provide better stakeholder traceability, and the ability to discover important requirements by comparing viewpoints. However, this theory has never been tested empirically. This paper reports on an exploratory case study of a key hypothesis of the viewpoints theory, namely that by creating separate viewpoint models to represent different stakeholder contributions, and explicitly merging them, important hidden requirements can be discovered. The case study compared two modelling teams using the i*notation to capture requirements for new webbased counselling services for a large charitable organisation. One team used viewpoints; the other did not. The conclusions include that viewpoint merging improves the understanding of the problem domain, but is very time consuming. The process of merging was more important than the merged product. The study also indicates a need for better model management tools, as both teams encountered dif- ficulty in managing large, evolving models.