A sensing mechanism of ethylenediamine (EN)-capped CdS nanowires (NW) towards silver and copper ions was revealed with a direct observation of the side product of the sensing event. The photoluminescence (PL) intensity of the CdS NWs was quenched by 59% and 35% in the solution containing the two metal ions. High-resolution TEM (HRTEM) image taken on the interface of the CdS NW and nanoparticle reveals two distinct sets of lattice fringes with interlayer spacing of 0.67 nm. The electron diffraction (ED) patterns verify the presence of Ag and Cu nanocrystals, with the dot patterns contributed by the single crystalline CdS NWs and ring patterns by the nanocrystals. The luminescent intensity of the CdS NWs of the excitionic emission decreases with increasing ion concentration and then reduces when the ion concentration reaches 800 μm.