In this article we conceptualise the public inquiry as a procedural tool and address the question of what makes a public inquiry an effective policy instrument. The issue of control is central to our arguments. In our conceptual work, we use control as a means of introducing the concept of the ‘catalytic procedural tool’ to better capture the variance in autonomy, location and function that can be associated with different inquiries. In our evaluative work, we use control as a means of analysing the effectiveness of an inquiry as a procedural tool, which centres on a capacity to build legitimacy and prospectively influence the implementation and institutionalisation of recommendations. We conclude by claiming that there is value in thinking about control as a means of understanding policy instruments because it can deliver insights into their effects once they leave the design table and enter a variety of technical, political and social environments.