The success or failure of international peacebuilding missions is predominantly evaluated in reference to interveners' ability to exercise their mandated authorities. To test the value of an empirically based analysis of authority-building processes in the course of such missions the article turns to the United Nations Transitional Authority in Cambodia (UNTAC, 1992-1993). In order to safeguard the country's stability many supported the idea of holding presidential elections in addition to the ones for the Constituent Assembly forming the new government. To organize such unforeseen elections UNTAC would have had to change its mandate as determined by the Paris Peace Accords (PPA). Based on extensive archival research the paper analyses the debate surrounding this proposal as a series of legitimacy claims that were selectively recognized and rejected. The article concludes that evaluations of peacebuilding missions are indeed too focused on interveners' authority to decide, while neglecting or underestimating challenges to their authority to interpret. This fosters a false sense of control over the direction of political processes.