Strategic behaviour (SB) is defined as the behaviour of stakeholders aiming to maximise their own goals and interests by influencing the outcomes in a decision-making environment. This behaviour is by nature included in the multi-stakeholder decision-making processes, as they use it to influence the process and align the outcomes with their own interests. This can result in imbalances and less-optimal outcomes of the decision-making processes. Decision Support Systems (DSS) are designed to improve decision quality. However, their effectiveness can be compromised when behavioural dynamics are not fully understood. This study aims to investigate how DSS influence SBs (including collaborative and competitive strategies, by using a specific DSS namely the Preference-Based Accommodation Strategy (PAS). PAS is a design and decision support system, which is designed to include the preferences of all the decision-makers. This research uses different qualitative methods within the pilot study at a large Dutch governmental real estate organisation. Different interviews were conducted with key decision-makers to understand the SBs in both traditional and PAS-supported environments. In addition to this, observations were conducted during the execution of the PAS decision and decision support system to analyse stakeholders’ interactions and behavioural patterns in real time. These insights were then analysed to identify how DSS (in this case PAS) influence competitive and collaborative strategies. The results of the analysis show that the current decision-making environments lack transparency and inclusivity, which contributes to competitive behaviour. However, within the PAS-supported environment, collaborative strategies were more prevalent. This is due to the fact that transparency and structured decision process within the PAS encourage collaborative decision-making. The PAS integrates open dialogue and transparent negotiation. Moreover, it creates an environment where cooperative strategies become more effective and rewarding. By addressing the gaps in the DSS field, this study provides insights into the use of specific decision-making environments, such as PAS, to mitigate SBs, more specifically competitive ones, and enhance collective decision quality.