In this study, we demonstrate that Mn2+ can greatly promote the Cr(VI) reduction by oxalic acid at pH < 5 via an induction period and a subsequent auto-acceleration process. The Cr(VI) reduction rate constant during the late auto-acceleration process was about 10 times that of the initial induction period. Characterization results revealed that this interesting two-step Cr(VI) reduction phenomenon was attributed to the in-situ generated Mn3+ by the oxidation of Mn2+ with Cr(VI) in the presence of oxalic acid during the induction period. The in-situ generated Mn3+ might complex with oxalate and Cr(VI) to produce a ternary complex, thus facilitating the electron transfer from oxalate to Cr(VI) to automatically accelerate the Cr(VI) reduction process. These findings shed insight into the possible roles of widely existed Mn ions (Mn2+ and Mn3+) and oxalic acid in the transformation of Cr(VI) in natural aquatic environment, and also provided an efficient way to remediate Cr(VI)-containing acid wastewater. 2017 Elsevier B.V. All rights reserved.