This article is part of the ongoing efforts to write a critical history of international arbitration in commercial and investment matters. It examines the ways in which the Spanish crown and its concessionaries set up a mechanism to settle legal disputes pertaining to the transatlantic slave trade. The transformation of asientos de negros from limited royal contracts to large-scale monopolies awarded to foreign chartered companies during the sixteenth, seventeenth and early eighteenth centuries was accompanied by the creation of an international commercial arbitration system. Why was this system set up, how did it work, and what was its faith? The overall aim of the article is to invite international lawyers to rethink the history of international arbitration and pay closer attention to the ‘private’ dimensions of formal and informal imperialism. It also attempts to bridge the historical investigation and contemporary commentary. In the conclusion, I argue that this study allows us, in a mirroring effect, to question the idea that today's dispute settlement mechanism was conceived as a means to ‘depoliticize’ international investment law. What the introduction of arbitration achieves is to place some fundamental questions out of sight. Today, as in the past, arbitrators work from within the system; their work rests on a series of unspoken – and yet highly political – premises about the organization of economic life and the distribution of values.