Thermal wet oxidation of gallium arsenide GaAs (wafers) and gallium nitride GaN (layers from metalorganic vapor phase epitaxy MOVPE and hydride vapor phase epitaxy HVPE) was carried out in N2 as a main gas and H20 as an oxidizing agent. Materials parameters and surface morphology were studied by means x-ray diffraction, ellipsometry, photoreflectance PR, micro Raman spectroscopy, optical microscopy and atomic force microscopy AFM. The lack of materials parameters or their wide range, especially refractive index, dielectric constant and their dependence of oxide's composition and structure constituted some problems during measurements. GaAs oxidation was more difficult as GaN oxidation, especially GaNfrom HVPE. IntroductionAIIIBV and AIIIN semiconductors compounds are wide known as materials for optoelectronic devices. They are used often also to construction high temperature and microwave devices or chemical gas sensors. In these applications dielectric layers are necessary. The lack of a reliable and cheap technology of oxide layers can limit developing and application of semiconductor compounds devices. There is a possibility to make and use their own oxidegallium oxide Ga2O3 gives the opportunity of manufacturing of many different devices with metal oxide semiconductor MOS structures e.g.: MOS capacitors, power metal oxide semiconductor field effect transistors MOSFETs, high mobility GaAs MOSFETs or gate turn-off thyristors and, probably, CMOS applications [1,2]. It can be possible to oxidize the metal electrode in GaN devices. The MOS-gate version of the HEMT has significantly better thermal stability than a metal-gate structure and is well suited to gas sensing [3,4,5]. High electron mobility in Ill-V compounds causes that it is important group of materials from which can be manufactured very speed electronic devices, especially by using oxides [6].There are several methods which lead to obtain Ga2O3 layers. Some of them allow to produce gallium oxide films with a good quality, for example by chemical or physical deposition [3,7,8], but in an expensive way. An anodic oxidation [9] gives layers without good parameters for devices fabrication. Thermal oxidation of AIIIBV and AIIIN compounds -dry [10,11] or wet [12,13] -is unfortunately not similar to silicon oxidation. It is caused by another structure -these compounds consist of two or more elements which variously react with oxygen and water. In addition AIIIBVs and AIIINs are very thermodynamically unstableespecially As and N have high partial pressures. In spite of these difficulties in many laboratories one carry on thermal oxidation studies because this technique is relatively cheap and can give good results. The most known is AlAs oxidation for diode lasers, electroluminescent diodes and detectors with Bragg reflectors [14].