The thermal annealing of 1.5 I~m thick SiO 2 layers deposited on Si with a High Density Plasma (HDP) has been monitored by Thermal Desorption Spectrometry (TDS), Fourier Transform Infra-Red absorption (FFIR) and Positron Annihilation Doppler Broadening (PADB). Two samples were prepared with different production conditions (substrate temperature, Ar flow, O2/SiH 4 ratio) in order to get oxide layers rich in Si-H bonds or rich in Si-OH bonds. The deposition temperatures of these layers were 545 K and 575 K, respectively. The formation of Si-H rich and Si-OH rich oxides was confirmed by FTIR absorption measurements. Desorption measurements showed that at a heating rate of 5 K/s release of hydrogen takes place around 1100 K. For the layer deposited at 575 K additional release of hydrogen is observed at 900 K. In both samples argon release is seen around 1200 K. The release of hydrogen at 1100 K is consistent with FTIR measurements at annealed samples. The absorption peaks associated with the Si-H and Si-OH bonds only disappear after heating above 1000 K. For the PADB measurements thermal anneals were carried out from 300 to 1300 K in steps of 50 and 100 K of 16 min duration. The positron measurements show different annealing behavior of the two oxides. It is observed that up to 1200 K the 545 K oxide shows a significant higher value for the defect parameter. The argon and hydrogen release is observed by an increase of the defect parameter at 1000 K.