Résumé : Despite its high potential to support decision-making, the role of Social Impact Assessment (SIA) within the policy process remains largely questioned. In this study, we analyse SIA’s feasibility in a context of real-world policymaking, by means of a case study on a legislative proposal on integration policy for immigrant newcomers in the Brussels-Capital Region, for which we evaluate the potential effects on poverty and social inequalities. We first analyse the policy process surrounding the policy proposal – a process that is often treated as a black box within SIA research. Understanding the factors that influence and determine the decision-making process, enables us to gain insight into SIA’s potential decision-support function(s). Second, we develop an approach to SIA that aims to fully exploit its potential to contribute to the functions of both instrumental and conceptual learning. For this purpose, we propose to introduce the approach of realist evaluation within the SIA-model, and to focus on evaluating the underlying policy intervention theory from the perspective of poverty and social inequalities.Finally, we illustrate this new approach and its added value by applying it to the legislative proposal on integration policy and analyse its contribution to policy-oriented learning.