Current political trends and scholarly research increasingly promote collaborative and participatory governance in multi-level systems as a way to more sustainable and effective environmental policy. Yet empirical findings as well as conceptual works from different academic fi elds remain ambiguous about this claim. This paper explores whether and to what extent the existence of multiple levels of governance affects the ability of participatory decision-making to deliver high quality environmental policy output and to improve implementation and compliance. To this end, fi ndings from the literature on multi-level governance, public participation and policy implementation as well as on complex systems are integrated in fi ve sets of hypotheses. In order to put these to a 'plausibility probe', a meta-analysis of 47 case studies from Northern America and Western Europe is conducted. These cases provide qualitative insights and allow for some generalization in the form of correlation analysis. The study fi nds that, predominantly, environmental preferences of the involved actors determine the environmental outputs (and outcomes) of decision-making. Further, face-to-face, but not mere two-way, communication appears to positively influence the ecological standard of decisions. The analysis also suggests that a highly polycentric governance system comprising many agencies and levels of governance yields higher environmental outputs than rather monocentric governance. However, correlations between governance effectiveness and decision-making scale, as well as policy delivery and institutional fit to ecosystem, could not be identified. The paper concludes by outlining pathways for more systematic comparative research on these pressing research questions. Copyright © 2009 John Wiley & Sons, Ltd and ERP Environment.