A one-step method to prepare core-shell nanoparticles and thus hollow nanospheres is reported. The process for the formation of core and shell took place during reaction. Once the core formed, it was covered with the shell substance produced in situ, and thus, the shell hindered the continued growth of the core. Based on this method, we readily obtained core-shell nanoparticles by choosing AgCl, CuS, or Fe(III) diethyldithiocarbamate (FeDEC)(3) as model core substances and the cogel from gelatin and gum arabic as the shell substance. High-resolution transmission electron microscopy (HRTEM) directly revealed the core-shell structure. TEM results showed the average particle sizes were under 100 nm, depending on the core substance and the concentration of substances producing cores. After removal of the core materials, hollow nanospheres resulted, which were directly observed by TEM. The observation further verified the core-shell structure. UV spectrophotometry also gave signals of coated structure and revealed high core content (51.1%) and nearly perfect coating (91.6%). (C) 2003 Wiley Periodicals, Inc.