In recent decades, the Organisation for Economic Co-operation and Development (OECD) has become a powerful forum for trade liberalization and regulatory harmonization. OECD members have worked to reconcile divergent national regulatory approaches, applying a single framework across sovereign states, in effect determining whose knowledge-making practices would guide regulatory action throughout the industrialized world. Focusing on US regulators, industry associations, and environmental groups, this article explores the participatory politics of OECD chemical regulation harmonization in the late 1970s to early 1980s. These efforts were conditioned by differential institutional access and resources among stakeholders who sought to shape regulatory knowledge rules. Facing competing European and US approaches to chemical data—a minimum “base set” of test data versus case-by-case determinations—OECD members chose the European approach in 1980. However, US regulatory politics shifted with the election of President Reagan, prompting industry associations to lobby the US government to block the agreement. Examining the micropolitics of these standards in the making, I demonstrate that while long-term structures advantaged industrial actors, ideological alignment with the US government precipitated their decisive influence. The case illustrates the importance of attending to the distinctive politics of international harmonization and the effects on transnational knowledge-making and regulatory intervention.