The influence of five different chemical pretreatments on the optical properties of the Si surface was investigated by means of nulling ellipsometry. The ellipsometric data are fit to an air-SiO2-interface-Si model. The results of the experimental data interpretation show that thickness and structure of the "interfacial layer" are changed drastically; the interfacial layer thickness increases and the interfacial layer optical properties approach the optical properties of crystalline Si. These changes in the interfacial layer indicate the crystalline structure improvement of the substrate surface.