Cation-exchange reaction between Ag + and Cd 2+ was employed to transform single-crystal Ag 2 Se nanowires into single-crystal CdSe nanowires as a case study of transformation from one chalcogenide to another one. The CdSe nanowires of $30 nm in diameter had a wurtzite crystal structure and showed preservation of both the morphology and single crystallinity of the Ag 2 Se nanowires. This synthetic route is useful in producing large quantities of CdSe nanowires with uniform diameters and exceptionally long lengths (>50 lm).