The following contribution zeroes in on the diverging responses that permeate international adjudicative practice pertaining to international disputes arising out of scientific controversies. Drawing on the idea that scientific fact-finding is as much a struggle for argumentative persuasiveness as traditional fact-finding and law-interpretation, this article identifies and critically evaluates four attitudes of international judges and arbitrators. It shows that, when it comes to scientific fact-finding, adjudicative bodies are in a constant flux between nihilism, protectionism and outsourcing. It further demonstrates that similar dynamics can be observed with regards to the weighing of scientific knowledge in cases when adjudicators decide to outsource it to experts. This article subsequently argues that when confronted with scientific fact-finding, international adjudicators are dealing with knowledge that is as unstable as the law and which brings them to make a choice between different types of reasoning or rationality. It ultimately makes the argument that the question of scientific fact-finding inevitably confronts international judges and arbitrators with a choice of epistemic rationality.