Optical second-harmonic generation is a sensitive tool for investigating symmetry and composition of crystalline surfaces. Vicinal silicon (111) surfaces are appropriate and technologically important objects to demonstrate the sensitivity of this technique. Slightly misoriented silicon (111) surfaces with respect to the precise crystallographic (111)-plane display a step structure which generates contributions of two- and fourfold symmetry to the polarization and orientation dependent second-harmonic signal in addition to the expected components with three- and sixfold symmetry. Chemical modifications of the clean surface influence the components of different symmetry in a characteristic way. Thermally grown and native oxide layers of a thickness of 1 nm can be distinguished. Copper contamination also changes the different second-harmonic signal components systematically. An increase of several signal components is observed for copper concentrations exceeding 10(11) cm-2. Relative values for the components of the surface-susceptibility tensor are derived using a model that takes into account only surface contributions to the second-harmonic signal. Thus, information about the surface topology and about bonding properties in the close vicinity of the surface are obtained from the second-harmonic signals.