Oxidation of cobalt, nickel, molybdenum and tungsten nanoparticles under prolonged exposure in the air was studied. Nanoparticles were obtained by chemical dispersion with the following reduction temperatures: 200-400°C for cobalt; 200-400°C for nickel; 700-800°C for molybdenum and 750-850°C for tungsten. Scanning electron microscopy, transmission electron microscopy, XRD and thermogravimetric analysis were used for nanoparticles characterisation. The average size of nanoparticles, specific surface area and the coherent scattering area were determined. The method for oxide film thickness calculation on the surface of the nanoparticles was proposed and confirmed its adequacy. Based on experimental data, the growth law of the oxide film on the studied nanoparticles was determined. The obtained results can be used in the certification of nanoparticles and evaluation of a highly dispersed metal powders shelf life.