The goal of this paper is to explain the link between interest groups and political parties in the parliamentary arena. According to the information logic approach, policy makers decide whether to invite interest organizations taking into account their capacity to provide high quality technical information about policy problems. In contrast, the persuasion approach—, emphasizes policy-makers are especially willing to invite their allies, avoiding their enemies as a means to reinforce their negotiation capacity in the parliamentarian debate. We argue the political party willingness or interest in inviting and interest groups to testimonee in the parliamentarian arena is explained not only by the expertise they can provide or by their, reputation and representativeness, but especially by their capacity to reinforce MPs policy positions about policy issues. To develop the argument we have developed a unique database, which contains information about which political party invites which interest groups to discuss a policy issue for the last four legislatures in Catalonia