The adoption of Efficient Consumer Response (ECR) by the grocery industry has become increasingly important in order to stay competitive. Due to the inter-organizational nature of ECR, its adoption involves various parties with different and conflicting objectives and requires the concerted effort of supply chain members. Thus, third party organizations may be important in ECR adoption, but the exact roles they play have received little research attention. Employing a multiple case study, this study uncovers five roles third parties play and extends a previously published model of ECR adoption to include third parties as mediators in the ECR adoption process. The results are important theoretically for understanding the complexities of inter-organizational systems adoption general, and practically to promote the growth of ECR in particular.