The aim of this work is experimental analysis potential of possibilities of application oxidation in r.f. (13 .56 MHz) plasma for the formation of ultrathin (< 6 nm) gate oxide layers in MOS devices. Process is performed in very low temperature (350°C) in typical planar reactor. The obtained oxidation rate allows relatively easy control of final oxide layer thickness with oxidation time in very wide range, i.e. from 2 nm to approx. 10 nm. The properties of the obtained layers and systems (silicon-oxide) were characterised by electrical methods using specially designed MOS test structures. Number of electro-physical parameters were determined, e.g. : characteristic charges (fixed and interface traps), critical voltage causing breakdown, defects densities, ... etc. The spectroscopic ellipsometry and XPS measurements allowed independent determination of the dielectric layer thickness and obtaining additional verification of the quality and composition of the ultrathin oxide layer. The influence of high temperature annealing on properties of the formed layers and systems were also investigated.