<p>Sustainability science aims not only to understand pressing challenges but also to generate positive effects that drive sustainable transitions. However, little is known regarding how well this field achieves the latter goal. To address this gap, we explore the sustainability effects of research projects that have been conducted over 20 years in five biosphere reserves (BRs). Document analysis and expert interviews with BR managers and researchers were conducted to answer: (i) To what extent have BR become model sites in which transdisciplinary sustainable science is implemented systematically to induce sustainability effects? (ii) What factors enable or hinder the creation of positive research effects? Findings reveal that transdisciplinary research is still uncommon in BR but that disciplinary, interdisciplinary, and other smaller projects can also have positive effects. At first glance, this diverse portfolio of effective research types appears surprising because, according to the interviewed experts, key research criteria/principles which promote research effects reflect core aspects of transdisciplinarity, such as the coarticulation of research interests or the involvement of practice partners. These contradictions encourage to rethink sustainability science in BR, thus highlighting the fact that effective science is embedded in productive, short- to long-term science–society interactions that must be established and maintained to systematically generate impacts. Although in larger projects, these interactions can be established within their project framework, for smaller projects, they must be facilitated by a generally beneficial socioscientific environment. BR are important for fostering these environments, thereby serving as long-term intermediaries and knowledge brokers between science and society.</p>