Metal nanoparticles were synthesized continuously in supercritical methanol (scMeOH) without using reducing agents at a pressure of 30 MPa and at various reaction temperatures ranging 150-400 degrees C. Wide angle X-ray diffraction (WAXD) analysis revealed that metallic nickel (Ni) nanoparticles were synthesized at a reaction temperature of 400 degrees C while mixtures of nickel hydroxide (alpha-Ni(OH)(2)) and metallic Ni were produced at lower reaction temperatures of 250-350 degrees C. In contrast, metallic silver (Ag) nanoparticles were produced at reaction temperatures above 150 degrees C while metallic cupper (Cu) nanoparticles were produced at reaction temperatures above 300 degrees C. Mixtures of copper oxide (CuO and Cu2O) and metallic Cu were produced at lower reaction temperatures of 250 degrees C. Scanning electron microscopy (SEM) showed that the particles size and morphology changed drastically as the reaction temperature increased. The average diameters of Ni, Cu and Ag particles synthesized at 400 degrees C were 119 +/- 19 nm, 240 +/- 44 nm, and 148 +/- 32 nm, respectively. The scMeOH acted both as a reaction medium and a reducing agent. A possible reduction mechanism in scMeOH is also presented. (C) 2010 Elsevier B.V. All rights reserved.