A new approach for the synthesis of Co-Fe Prussian blue nanoparticles with controlled size has been developed in the present work. Mixture of formamide and water was used as a reaction medium for the chemical synthesis of the nanoparticles at room temperature. It has been found that the size of nanoparticles can be controlled by varying the volume ratios between formamide and water. Powder X-ray diffraction, transmission electron microscopy, diffuse reflectance UV-vis absorption spectra, and nitrogen adsorption/desorption isotherms were employed to characterize the products. The optical properties of the nanoparticles depending on particle size were observed. (c) 2007 Elsevier B.V. All rights reserved.