Formic acid decomposition on noble metals is considered to be a potential method to produce CO-free hydrogen at near ambient temperatures. However, the reaction mechanism, as well as the key points, for HCOOH decomposition on noble metals in aqueous solution remains unclear at microscopic level. In the present work, we employed density functional theory (DFT) calculation to investigate HCOOH decomposition in gas and aqueous phases on four common noble metals (Pt, Pd, Rh, and Au). Based on the present theoretical calculation results and experimental results being available in literature, two reaction pathways were proposed to understand gas- and aqueous-phase HCOOH decomposition on the noble metals. The key points that determine the activities of the metals toward HCOOH decomposition into CO2 and H-2 in aqueous solution are clarified. Furthermore, the proposed reaction mechanism can be well extended to interpret the excellent activity of Ag-Pd core-shell bimetallic catalyst for HCOOH decomposition in aqueous solution. It is expected the present reaction mechanisms would enable us to rationally design more active heterogeneous catalysts for HCOOH decomposition into CO-free H-2 at relatively low temperatures. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.