Nanosized nickel particles produced in aqueous sol form were prepared by gamma radiolysis of nickel sulfate solution, using gelatin as a stabilizer. The average size of these particles was estimated from the width of the powder X-ray diffraction pattern to be approximately 5 nm, The magnetization measurements were carried out from room temperature to 5 K. The ferromagnetic character of these particles was studied at 5 K. The room temperature catalytic activity of this nickel sol was demonstrated by microcalorimetry measurements of the heat evolved during chemisorption of hydrogen, and the reaction of hydrogen with oxygen and ethylene at room temperature was investigated.