We have developed a simple method to synthesize size-controlled amorphous selenium nanoparticles by chemical reduction at room temperature. The particle size control can be achieved by adjusting the key element of this synthetic system, the sodium thiosulfate as the reducing agent with the aid of the surfactant stabilizers. Mean particle diameters can be controlled well in the range from 20nm to the sub-micrometer regime. A clear trend in the absorption spectra of these particles is the red-shift that occurs as the particle sizes increase; this effect is analogous to the confinement effect of semiconductor nanoparticles and reflects in the color change from yellow to red. This spectral evolution is in very good agreement with the classical Mie formalism predictions. © 2005 Elsevier B.V. All rights reserved.