The article discusses the parameters conditioning the chair's effectiveness in international multilateral negotiations. Building on existing accounts of Presidential functions, an analytical typology of these parameters is provided, elaborating on the direction and magnitude of their impact. Conditioning parameters are clustered into three categories: the broader international environment in which the chair operates and the issues with which (s)he is called to deal; organization-specific features of an institutional and political nature; and the chair's personal skills and country-of-origin attributes. The potential for effect of these parameters is illustrated by reference to the chairmanship of the UN Security Council (SC). The origins, institutional features and main tasks of the Council are discussed, as well as the role of its chair as an agenda manager and broker. It is argued that the SC President's role well exceeds the formal tasks that are stipulated in the Rules of Procedure. Although mainly procedural in nature, these tasks may have important political implications, stressing the informal component of Presidential intervention. The conditioning parameters identified have a catalytic impact upon and account for the great variance in SC chair performance and the effectiveness of the chair's intervention.