Water-soluble cubic structure Ag(2)Se (alpha-Ag(2)Se) nanocrystals smaller than 5 nm can be obtained by cation-exchange reaction at room temperature, using water-dispersed ZnSe nanocrystals as precursors, which is achieved by controlling the injection speed of AgNO(3) solutions via a syringe pump in the presence of the stabilizer of trisodium citrate. Meanwhile, the thermal stability of the product Ag(2)Se nanocrystals is studied. The results show that the mean sizes and shapes of the precursor ZnSe and product Ag(2)Se nanocrystals are similar, and Se anion sublattices between them are topotaxial. In addition, no phase transition is observed for the product Ag(2)Se (cubic structure) nanocrystals below 180 degrees C. The present synthetic method based on cation-exchange reactions can also be applied to the syntheses of PbSe and CuSe nanocrystals.