Tin oxide (SnO2) nanowires (NWs) with diameters of 50 nm, lengths up to 100 µm and a tetragonal rutile crystal structure have been grown by low pressure reactive vapour transport on 1 nm Au/Si(001). The free carrier density of the SnO2 NWs measured by THz absorption spectroscopy was found to be n = (3.3 ± 0.4) × 1016 cm−3. Based on this we have determined the one-dimensional (1D) sub-band energies, overall charge distribution and band bending via the self-consistent solution of the Poisson–Schrödinger equations in cylindrical coordinates and in the effective mass approximation. We find that a high density of 1018–1019 cm−3 donor-like defect related states is required to obtain a line density of 0.7 × 109 close to the measured value by taking the Fermi level to be situated ≈0.7 eV below the conduction band edge at the surface which gives a surface depletion shell thickness of 15 nm. We discuss the origin of the donor-like states that are energetically located in the upper half of the energy band gap as determined by ultrafast, time-resolved absorption–transmission spectroscopy.