This paper describes a usability study of a responsive website prototype. A heuristic evaluation was conducted with three evaluators with a background in HCI, and user tests with four potential users were conducted as a control method to test the relevance of the heuristic evaluation result. The study showed that the website prototype failed the visibility of system status, user control and freedom and error prevention heuristics in the heuristic evaluation. However, the users responses contradict that result by showing that the users had no problems with those areas in the user tests. In fact, the users detected no usability problems at all. This could mean that those heuristics are not relevant for a responsive Web 2.0 site and needs to be removed when conducting a heuristic evaluation of a responsive website in the future, and that designing for responsiveness leads to higher usability. This study shows some indications but more testing needs to be done, preferably on several fully developed responsive websites, to be able to make any real conclusions about the usability of a responsive website and the relevance of the heuristics used in this study.