Water has long been used and regarded as a solvent in the synthesis of noble-metal colloids, but its role remains largely unexplored in terms of reactivity and functionality. We demonstrate that water can serve as a reducing agent for the synthesis of platinum (Pt) nanocrystals. Starting with only two components, i.e., K2PtCl4 and deionized water, we successfully synthesized Pt nanocrystals with an average size of 1.9 nm, without the aid of any exotic reducing agents as well as organic capping agents or stabilizers. Because of small particle sizes, these Pt nanocrystals exhibited a high activity toward the oxygen reduction reaction. (C) 2014 Elsevier B.V. All rights reserved.