We present an example of an interparticle reaction between atomically precise nanoclusters (NCs) of the same metal, resulting in entirely different clusters. In detail, the clusters [Ag12(TBT)8(TFA)5(CH3CN)]+ (TBT = tert-butylthiolate, TFA = trifluoroacetate, CH3CN = acetonitrile) and [Ag18(TPP)10H16]2+ (TPP = triphenylphosphine) abbreviated as Ag12 and Ag18, respectively, react leading to [Ag16(TBT)8(TFA)7(CH3CN)3Cl]+ and [Ag17(TBT)8(TFA)7(CH3CN)3Cl]+, abbreviated as Ag16 and Ag17, respectively. The two product NCs crystallize together as both possess the same metal chalcogenolate shell, composed of Ag16S8, making them indistinguishable. The occupancies of Ag16 and Ag17 are 66.66 and 33.33%, respectively, in a single crystal. Electrospray ionization mass spectrometry (ESI MS) of the reaction product and a dissolved crystal show the population of Ag16 and Ag17 NCs to be in a 1:1 and 2:1 ratio, respectively. This suggests selective crystallization in the cocrystal. Time-dependent ESI MS was employed to understand the formation of product clusters by monitoring the reaction intermediates formed in the course of the reaction. We present an unprecedented growth mechanism for the formation of silver NCs mediated by silver thiolate intermediates. © 2019 American Chemical Society.