Quartz crystal microbalance with dissipation (QCM-D) analysis of the hexa-L-Leucine (PLL)-catalyzed epoxidation of chalcone gives in-situ experimental evidences which demonstrate that the reaction proceeds mainly via the formation of a PLL-bound hydroperoxide complex followed by the reversible addition of chalcone. The observations offer an alternative rationalization for the viability of the preferred catalytic pathway. (C) 2016 Elsevier Inc. All rights reserved.