Silica gel coatings prepared by the sol-gel method were subjected to low-temperature plasma treatments in air and argon. This was found to give rise to the formation of a dense outer layer, whose thickness increased with the duration of the treatment and decreased when the pressure in the plasma chamber was reduced. The formation of a dense layer in the coatings was confirmed from measurements of the overall thickness and the refractive index by ellipsometry, and also from TEM examination and light transmission experiments. The coatings were found to contain an uppermost layer of alumina, which was formed through secondary sputtering of aluminium from the electrodes in the plasma chamber. The concentration of alumina in the mixture with silica decreased through the thickness of the coating and became zero at a distance slightly smaller than the overall thickness of the dense layer. The thickness of the dense layer was found to be slightly higher with argon than with air plasma treatments. Whereas the aluminium concentration through the thickness of the coating was the same for the two types of treatment, a carbon residue was found only in the case of argon plasma treatments. The com position of the underlying silica layers was found to correspond to SiO1.6.