Ambient, structure-and topology-preserving chemical reactions between two archetypal nanoparticles, Ag 25 (SR) 18 and Au 25 (SR) 18, are presented. Despite their geometric robustness and electronic stability, reactions between them in solution produce alloys, Agm Aun (SR)18 (m+n=25), keeping their M25 (SR)18 composition, structure and topology intact. We demonstrate that a mixture of Ag25 (SR)18 and Au25 (SR)18 can be transformed to any arbitrary alloy composition, Agm Aun (SR)18 (n=1-24), merely by controlling the reactant compositions. We capture one of the earliest events of the process, namely the formation of the dianionic adduct, (Ag25 Au25 (SR)36) 2-, by electrospray ionization mass spectrometry. Molecular docking simulations and density functional theory (DFT) calculations also suggest that metal atom exchanges occur through the formation of an adduct between the two clusters. DFT calculations further confirm that metal atom exchanges are thermodynamically feasible. Such isomorphous transformations between nanoparticles imply that microscopic pieces of matter can be transformed completely to chemically different entities, preserving their structures, at least in the nanometric regime. © 2016 The Author(s).