Recent advances in technology have made truly open and accessible government significantly more realisable. One of the ways in which governments are using this technology is in the implementation of online portals that allow open (i.e., public and unrestricted) access to data and use of data. Such portals can be used by citizens and professionals to facilitate improved decision-making across a wide range of areas, from car-parking to promoting entrepreneurialism. However, the existence of portals per se is not enough. To maximise their potential, users must also feel that they are both accessible and usable. To gain insights into the current state of usability of OGD portals for professionals working in data-related areas, a comparative study of the portals of the G7 group was carried out, using a mixed methodology. This is the first specific comparison of these portals for such users, as well as the first study to add a user-centred qualitative dimension to the research. The study’s findings showed that the G7 countries are not maximising the potential of their portals or collaborating effectively. Addressing these issues, and building better cross-national consistency, would help to improve the value delivered by investment in OGD portals. The study also further supported an existing user-centred, heuristic evaluation framework for application to a more specific user group, as well as more generally.