Some detailed investigations recently carried out on plasma-polymerized organosilicon films have revealed a number of phenomena which may be related to a phase transition taking place at ca. 270 K. The existence of this effect is confirmed by measurements of the temperature dependence of the thermal expansion coefficient of these films, by DSC measurements, and by measurements of the temperature dependence of the dielectric constant. It was found that the observed phase transition effect appears to be related to the presence of water molecules in the film volume. The water molecules form aggregates strongly bound to the film structure and stay there even under vacuum and at high temperature (to ca. 450 K).