The method of field effect in electrolytes was used to determine the surface electrical properties and band parameters in surface layers of semiconductor solid solutions (TlBiSe₂)_1-x-(TlBiS₂)_x at room temperature. The dispersion law, the effective mass of electrons in the conduction band, the ionized donor impurity density, and the Fermi level are determined. The experimental and theoretically calculated capacitance-voltage characteristics are compared.