Colloidal silver nanoparticles were synthesized in supercritical ethane at 80 degreesC and 80-120 bar, with methanol as cosolvent. Solvated electrons, produced by a pulse of 20 MeV electrons, reduced the silver ions. The time-resolved technique of pulse radiolysis was employed to characterize the reduction products and colloidal metallic particles. The absorption spectra of small silver clusters (Ag-2(+), Ag-3(+), Ag-4(2+), etc.) were detected at short times after the pulse. Colloidal metallic silver particles were identified by their characteristic plasmon absorption at 1-10 s after the pulse. Colloidal particles are stable for hours in supercritical ethane. The particles are less than 10 nm in diameter. Their size was determined using transmission electron microscope after precipitation from the solution.