Starting from silicon on insulator substrates, we show that a thermal treatment (in the 600–900 °C range) induces the creation of silicon islands. To characterize the island formation as well as the initial silicon layer thickness, we use in situ Auger electron spectroscopy analysis in an ultrahigh vacuum chamber. The island size and density are studied with an ex situ atomic force microscope. We show that the formation temperature of the islands increases from 575 to 875 °C as the initial silicon layer thickness increases from 1 to 19 nm. For the 1 nm thickness, the minimum island size is reached (semispherical shape with a 16 nm diameter). The phenomena involved in the island formation are discussed and the study of the variations of the calculated stress tensor (IMPACT software) as a function of the thermal treatment explain the behavior of the top silicon layer