Hydrolysis, condensation and polymerization of γ-glycidyloxipropyltrimethoxysilane was studied under different reaction conditions and yields a hard coating material for polyethyleneterephthalate substrates. The progress of hydrolysis was followed by 29Si-NMR spectroscopy and Karl Fischer filtration. Hydrolysis proceeds very slowly at pH 5.5, but can be enhanced at pH 9. Independent from submitted water and pH the solutions only one half of hydrolyzable methoxy groups can be removed. Condensation and polymerization were investigated on coated substrates and start during thermal curing. The formation of the polysiloxane network takes place by elimination of methanol. Polymerization of the epoxy groups to polethyleneoxide units is achieved above 100°C using a special catalyst. The properties of the cured coatings depend on the free water and glycol content in the solution. Minimizing the amount of both components leads to an ORMOCER coating on polyethyleneterephthalate which features a good abrasion resistance in combination with a high flexibility.