This is the published version of a paper published in Project Management Journal. Citation for the original published paper (version of record): Miterev, M., Engwall, M., Jerbrant, A. (2017) Mechanisms of isomorphism in project-based organizations. Project Management Journal, 48(5): 9-24 Access to the published version may require subscription. N.B. When citing this work, cite the original published paper. Permanent link to this version: This article challenges the dominant assumption of goal rationality behind temporary organizations' design in project-based organizations (PBOs). While extant literature posits that organizations strive to select the most appropriate project arrangements to fit particular task requirements at hand, findings from an in-depth ethnography-inspired case study suggest that projects in PBOs tend to imitate each other's structures, strategies, and practices with little consideration of the potential performance effects. Building upon the new institutionalism, this article conceptualizes the PBO as an organizational field of temporary and permanent organizations embedded in wider organizational and institutional fields and explicates isomorphic processes among temporary organizations in PBOs.