A method of reacting pretilt-inducing organic groups with an orientation layer of an organic polymer which is provided on a substrate, the orientation layer being intended to induce a molecular ordering in the plane of the substrate of molecules of a liquid crystalline material lying against the orientation layer, and the pretilt-inducing organic groups being intended to induce a pretilt angle between the substrate and molecules of the liquid crystalline material in contact with the pretilt-inducing organic groups on the orientation layer, characterized in that a precursor compound of the organic group is vaporized and in that the substrate carrying the orientation layer is exposed to the vapour formed in this process.