Questions of justice are often at the heart of international negotiations. While previous research has established a link between justice and the effectiveness of negotiations, the mechanisms behind justice behaviour in international negotiations remain understudied. Against this background, this paper will investigate the question: What factors determine which justice principle negotiators invoke or agree to in international environmental negotiations? In order to answer this question this study will apply a controlled comparison applying the congruence method to five pairs of cases, covering a broad range of environmental negotiations. Within each pair, one factor theorized to play an important role in shaping justice behaviour in environmental negotiations will be analysed. The factors are 1) setting of the negotiations, 2) power balance between the parties, 3) scientific (un)certainty, 4) domestic constituencies, and 5) common crisis experience. The findings are expected to discern factors determining which justice principles negotiators invoke and under what conditions agreement on justice notions is promoted. Better understanding of what motivates negotiators’ choices of justice principles and their mutual acceptance can help to strengthen the link between justice and effectiveness of negotiations. In this way, the paper’s findings will be of relevance from both a research and policy perspective.