The process of thermal decomposition of SiOx layers prepared by magnetron co-sputtering of Si and SiO2 on Si and quartz substrates is studied by Auger and secondary ion mass spectroscopies. It is found that high temperature annealing of the layers causes a Si-depleted region near the layer/substrate interface. It is shown that the formation of this region does not depend on the type of substrate but depends on the content of excess Si and is observed at high content of excess Si. When the excess Si content decreases, the Si-depleted region at first smears and then disappears. The mechanism of SiOx decomposition and possible reasons for the appearance of the Si-depleted region are discussed. (C) 2007 Elsevier B.V. All rights reserved.