When do members of civil society view international election observers as legitimate? Motivated by recent work on the legitimacy of international organizations, we evaluate what type of information affects non-governmental organizations’ (NGOs) beliefs about international election observer groups, which include both intergovernmental organizations (IGOs) and international non-governmental organizations (INGOs) that seek to exercise authority, often regarding the same elections. Specifically, we examine the effects of two different types of information: information about the observers’ objective substantive features versus information that serves as heuristic shortcuts. Three survey-based experiments – one in Kenya and the others global – prime NGO respondents with information about both real and hypothetical election observer groups in ways intended to affect their votes for which organizations should be invited to observe the next election in their countries. In general, the primes about the objective substantive sources of legitimacy beliefs failed to produce consistent, measurable changes in responses among NGOs across both the hypothetical and real-world observer groups. That is, telling NGOs about the qualities of the organizations work failed to change perceptions. What mattered instead, however, was an organizations’ prominence or type, features that likely served as heuristic shortcuts. The findings, however, varied depending on whether we used hypothetical or real organizations. With hypothetical organizations, NGO respondents preferred other NGOs, suggesting an isomorphism heuristic. Conversely, with real organizations NGO respondents preferred more prominent and well-known intergovernmental organizations. This suggests that the isomorphism and prominence of observer organizations can drive legitimacy beliefs. Given the differences between using real versus hypothetical organizations, however, it also cautions against using hypothetical actors in survey experiments.