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Surface‐related effects in undoped and Cr‐doped semi‐insulating (SI) liquid‐encapsulated Czochralski (LEC) GaAs, as seen in Hall effect and resistivity measurements, are shown to be caused by mixed conduction due to a p‐type surface space‐charge layer, as predicted by theoretical calculations employing a surface‐interface state model recently developed to explain surface‐potential changes in epitaxial GaAs. Wet chemical treatments with ammonium hydroxide were used to induce especially large surface effects and are shown to form a substantial hole concentration at the surface. The excellent agreement between the theoretical results and experimental data provides strong support for the surface‐interface state model used and demonstrates its utility as the first ‘‘working’’ mathematical model of surface‐interface states in GaAs. In addition, wet chemical treatment in hydrogen peroxide is shown to minimize the surface layer and thereby allow accurate as well as stable and repeatable electrical characterization of the n‐type bulk.