The in-situ IR technique was employed to monitor the crystallization process of Zr-fumarate (Zr-fum) MOF under real synthesis conditions. The results evidenced the participations of both modulator acid (HCOOH) and solvent (DMF) during the structural formation of Zr-fum MOF. This observation will contribute to the quest for understanding of the formation of structural defects in MOFs, which is a topic that is currently under intense investigation. (C) 2017 Elsevier B.V. All rights reserved.