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SnO2 doped TiO2 materials have received considerable attention due to their high dielectric permittivity (on the order of 10,000) and low loss factor (< 0.1) over a broad temperature/frequency range. In this work, a SnO2–TiO2 nanocomposite was successfully synthesized by a single solvent process. The synthesized samples were characterized by X-ray diffraction, high resolution scanning electron microscopy, UV–Vis spectroscopy, photoluminescence and impedance analysis. The high dielectric permittivity of SnO2 doped TiO2 has been attributed to a special defect-dipole structure. Such dielectric behavior has been observed in this work in hydrogen-reduced TiO2–SnO2 crystals, where the colossal dielectric permittivity and the low loss factor of ~ 0.1 exist in the temperature range of 20–300 °C (and frequency at 10 ~ 105 Hz). Based on the investigations on the phase microstructures, the elemental valences, impedance spectroscopy and dielectric response, the colossal dielectric permittivity of the hydrogen-reduced TiO2–SnO2 crystals was thought to be associated with the electronic relaxation polarization mechanism. This research further revealed that the weak-binding electron is the most critical factor for the high dielectric permittivity in TiO2–SnO2 system, while a special defect-dipole structure may not be a preliminary determinant. [ABSTRACT FROM AUTHOR]