The mechanism of reduction of AuCl4-/AuCl3OH- by BH4- was analyzed by density functional theory (DFT). The results point out that Au-atmos(0) are not intermediates in the process. The derived mechanism differs considerably from that reported for the analogous process involving the reduction of Ag(H2O)(2)(+) by BH4-. Thus, though both processes follow the Creighton procedure, the detailed mechanism differs significantly. For Au, the agglomeration starts with AuH2-, whereas for Ag, it starts with (H2O)AgH. Stopped-flow measurements support the complicated mechanism.