Metallic Ag nanoparticles have been converted to Ag2Se nanoparticles at ambient temperature and open atmosphere by UV photodissociation of adsorbed CSe2 on the Ag core surface. The photolysis could be prevented at any stage yielding Ag@Ag2Se core-shell structures of different thickness. Depending on the initial Ag nanoparticle size, either hollow or filled nanocrystals of Ag2Se could be prepared. The Kirkendall effect has been proposed to account for the formation of hollow nanoparticles. A coated-sphere Drude model has been used to explain the redshift of the Ag plasmon band as a function of the Ag2Se shell thickness as well as to provide the first estimates of the wavelength-dependent dielectric function of Ag2Se. This photochemical method might be especially promising for carrying out a direct room-temperature phototransformation of metallic into semiconductor nanostructures already assembled on surface templates.