Current research struggles to illuminate significant learning outcomes of role-play simulations of decision-making. This is particularly the case for more complex simulation environments such as Model European Union or Model United Nations. Previous research has shown that self-efficacy for negotiating is an important learning outcome that seems to increase over time within one simulation experience. However, results also show important individual variations in students’ development of self-efficacy for negotiating. Both contextual aspects (e.g. feedback) and individual characteristics (e.g. prior success) are known for influencing the development of self-efficacy. It is however unclear to what extent these or other aspects relate to stimulating or inhibiting conditions within a simulation environment that aims for fostering self-efficacy for negotiating. To increase our understanding of the simulation process, this paper presents a case study conducted during a four-day model European Union simulation, focusing on the conceptual development of negotiating and on students’ self-efficacy for negotiating as learning outcomes. Using a single holistic longitudinal case study design, four students of the same university participated in this study. Data were triangulated using both in-depth interviews on four time-points (1pre-, 3 post-interviews) and reflection reports. The following questions were posed. How did students experience the simulation environment? How are these experiences related to changes in their self-efficacy for negotiating? How are these experiences related to their conceptual development of ‘good’ negotiating? Results show variation in learning outcomes among students both for self-efficacy for negotiating and for how they conceptualise ‘good’ negotiating. A cross-case analysis shows the importance of the relational component, how other simulation participants may or may not contribute to the development of self-efficacy for negotiating by creating conditions that foster negotiation skills and/or by performing in a particular way e.g. to what extent is their performance related to reality, if and how is feedback provided.