This article explores the role of expertise in decision-making by studying the influence of non-state actors in the negotiations of the Convention to Combat Desertification. Actors possessing issue-relevant knowledge and the skill and judgment in how to use this knowledge – often referred to as experts – are consulted on specific issues and may exert influence over the result of negotiating processes. Conventional wisdom suggests that since they are requested to provide advice, scientific advisers are likely to wield high levels of influence at certain moments in environmental negotiations. There is also a growing literature that suggests that NGOs have increasing influence in such negotiations. This article examines both these propositions and finds that, in the case of the desertification negotiations, the formally appointed scientific advisory body – the International Panel of Experts on Desertification (IPED) – had insignificant influence on the process and outcome of the negotiations. This was due to IPED's perception of its own role, a preemption of IPED's functions, and the mandate and design of IPED. NGOs, however, exerted a high degree of influence because of the participatory approach promoted in the Convention, the composition of the attending NGOs, and the supporting environment in the negotiations. The article concludes by suggesting some implications of these findings for international environmental negotiations.