Silicon nanoparticles were prepared in a homemade apparatus by means of a dc sputtering method in which the condensates were collected directly from the cold surface of a liquid nitrogen trap. They were dispersed in 2-propanol under ultrasonic agitation, and dried in the atmosphere. The particles were found to compose of tiny silicon crystals and were only mildly oxidized. Various samples were prepared with different annealing times and temperatures in ultrahigh vacuum. XPS results show that, in the particles, the Si-O bonds of the Si4+ state are the most stable, followed next by the unoxidised state Si-0. The intermediate oxidation states are the least stable; they exist only at sufficiently low temperatures (300 degreesC or lower) and are converted to either Si-0 or Si4+ at higher temperatures.