Social and institutional diversity ("diversity" hereafter) are important dimensions in collaborative environmental governance, but lack empirical assessment. In this paper, we examine three aspects of diversity hypothesized in the literature as being important in collaborative forms of environmental governance-the presence of diverse actors, diverse perspectives, and diverse institutions. The presence of these aspects and formative conjectures were empirically considered using a mixed methods approach in four biosphere reserves in Sweden and Canada. We found that the diversity of actors involved and domains of authority varied among cases, that stakeholder perspectives were highly diverse in all cases, and that institutional variety (in terms of strategies, norms, and rules) was evident in all cases, but differed among them. Empirical support from the cases further affirms that diversity contributes to the ability to engage with a broader set of issues and challenges; diversity contributes to novel approaches to solving problems within the governance group; and diversity contributes to the flexibility of the group involved in governance in terms of addressing challenges. One conjecture, that diversity decreases the efficiency of governance in decision-making and responding to issues, was not supported by the data. However, our analysis indicates that there might be a trade-off between diversity and efficiency. The findings highlight differences in the ways in which diversity is conceptualized in the literature and on the ground, emphasizing the pragmatic advantages of actively seeking diversity in terms of competencies and capacities.