To determine the polymerization profile of a layer of polymeric material, particular a coating of an optical waveguide, a source sends a light radiation towards the layer under test at different angles of incidence, and both the portion of radiation reflected by the layer and the portion transmitted by the layer are converted into electrical signals. The electrical signals are supplied to devices for measuring their intensity, and a processing system processes the intensity values to obtain a value of the refractive index at a different depth inside the layer and to obtain the degree of polymerization at each depth by comparison with the values of the refractive index of a precursor of the polymer and of the fully polymerized material.