The four-step mechanism by which transition-metal nanoclusters or bulk-metal films self-assemble from metal salts under reductive conditions has been discovered. The presence of two autocatalytic steps in the same reaction scheme--double autocatalysis--is the key to the sharp "turn-on" feature after an induction period observed in the signature kinetic curves. Predictions of the new mechanism that are tested experimentally include the following: that low concentrations and high temperatures will favor nanoclusters over bulk-metal film formation; that bulk-metal is formed in some, if not many, literature syntheses reporting only Pt(0) nanoclusters; and that added ligands are one key to turning on the new mechanism. Particle-size-dependent metal-ligand bond dissociation energies are another implication from this mechanistic work.