Highly fluorescent and color tunable AgInS2 (AIS) and (AgInS2)x(ZnS)1-x (AIZS) quantum dots (QDs) were prepared via a facile aqueous-phase synthesis using AgNO3, In(NO3)3, Zn(OAc)2, and Na2S as precursors and 3-mercaptopropionic acid (3-MPA) as ligand. Produced AIZS QDs exhibit a small diameter (ca. 2.1 nm) and a cubic structure. Ag-3-MPA and In-3-MPA complexes formed during the preparation of AIS cores were found to play a key role on the fate of the reaction, and an atypical blue-shift of the photoluminescence emission was observed with the increase of the Ag/In ratio. The photoluminescence quantum yield (PL QY) of AIS QDs is modest but increased markedly after the alloying and shelling with ZnS (up to 65%). Size and composition-selective precipitations allowed to separate up to 13 fractions of AIZS QDs with exceptionally high PL QYs (up to 78%), which is the highest value reported for AIZS QDs prepared in the aqueous phase. These high PL QYs combined with their good colloidal stability and photostability make AIZS QDs of high potential as cadmium-free fluorescent probes for various applications like bioimaging or sensing.