Thermal decomposition of plasma-polymerized hexamethyl-cyclotrisilazane (PP-HMCTSN) thin films was studied by using various techniques. Pyrolysis-gas chromatography investigation of thermal decomposition products and IR examination of the polymer films showed that methylsilyl groups present in the monomer were highly susceptible to form disilymethylene and disilyethylene cross-linkages under plasma conditions. A three-stage mechanism of the thermal decomposition found by thermogravimetry proved a great complexity of the thermal reactions which occur during this process and, in turn, provided strong evidence for a very complex structure of plasma polymer. Electron microscopy investigation revealed a marked effect of the thermal decomposition process on the film microstructure. The trend observed in the morphological changes was consistent with kinetics of this process. The high temperature thermal treatment reduced the organic content of polymer film, correspondingly increasing its inorganic character. Films produced with this process were of unusually interesting properties, displaying strong adhesion to metal substrates and high resistance to corrosive agents.