In recent decades private actors have become increasingly important in the way sustainability challenges are dealt with and agricultural sectors are regulated. The so-called roundtables are examples of this private governance trend. A roundtable is a private arrangement with the aim to improve the sustainability of a specific global commodity chain. It is partnership where only private actors - businesses and non-governmental organizations - have decision-making power. In the literature two major perspectives towards the study of partnerships can be observed. Actor approaches explain the success (or failure) of partnerships in terms of managerial factors, while institutional approaches point to the societal context for explaining their success or failure. However, little is known about the relative importance of these (clusters) of factors that facilitate or hinder the development of partnerships or their ability to change global commodity chains. This paper combines an institutional with a behavioural approach by looking at behavioural (building trust; creating collaborative advantages; constituting a rule system; and changing a market) and institutional (the structure of the commodity chain; domestic drivers; the role of lead firms; product visibility to consumers; and NGO pressure) factors. The relative importance of these (clusters of) factors and there connections are established through a comparison of the development of two roundtables: the Roundtable on Sustainable Palm Oil and the Round Table on Responsible Soy. The outcome of the paper is a new model that helps to explain the development of private governance initiatives and their ability to change commodity chains.