D.c. electric field induced second-harmonic generation (SHG) spectroscopy of the Si(001)-SiO2 interface was studied both experimentally and theoretically. To describe the experimental results the general phenomenological model of d.c. induced SHG in centrosymmetric semiconductors is developed, taking into account surface and bulk field dependent as well as field independent contributions to the non-linear polarization. The solution of an inverse problem within this model of d.c. induced SHG allows all surface and bulk contributions to the non-linear response from an Si(001)-SiO2 interface to be distinguished.