In EU democratic deficit debate inter-parliamentary cooperation has gained attention as it could link the two levels of representation, the European Parliament and the national parliaments. The aim of this paper is to explain motivation for inter-parliamentary cooperation between national parliaments and the European Parliament. For what reasons do individual MPs pro-actively engage in inter-parliamentary cooperation? The paper takes a rational choice perspective developing expectations on potential benefits and limits of in inter-parliamentary cooperation. It developes four role types which account for diverse interests depending on MPs national, institutional, and party affiliation. Empirically, it is based on interviews with parliamentarians and administrators from seven parliaments, and draws on expert surveys on Denmark, Germany, the Netherlands, Poland, Portugal, and Slovakia, as well as the European Parliament. The advantage of the study lies in its comparative perspective. It allows testing for commonalities and difference in the incentive structure of MPs from member states of different size, length of EU membership, and geographical location, complemented by the supranational perspective of the European Parliament.The paper finds that MPs with pro-European stance, affiliated to an opposition party which is embedded in an EU party family, and who come from parliaments with week scrutiny rights within the domestic arena, are most willing to participate in inter-parliamentary cooperation. The findings on potential and challenges for inter-parliamentary cooperation are assessed in their implications for the democratic deficit of the EU.