Second-harmonic generation (SHG) was used to investigate chemically modified surfaces of Ge(111). Chemical modification was achieved by wet-chemical covalent binding of decyl and sulfur directly to the Ge interface after oxide stripping. Chemical modification of the interface substantially changes the second harmonic response. The decyl and sulfur terminations are stable in ambient during several weeks, as judged by SHG and XPS measurements. The SHG rotational anisotropy patterns were analyzed to estimate the relative values of the nonlinear susceptibilities describing the surface and bulk response. The choice of fundamental/SHG polarization combinations for accessing various nonlinear coefficients is presented. The factors affecting the relative values of the surface-to-bulk contributions to SHG and their changes upon chemical modification of the surface are discussed. In particular, it was found that the higher the electronegativity of chemically attached species, the higher the contribution of the surface-originating nonlinear terms to the overall response. Also, it was found that the relative contribution of surface versus bulk to SHG is different for different polarization combinations: the surface contribution to the p-in/p-out response is the greatest.