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Abstract: The spectral response has been used to evaluate the chemical cleaning for Ga1−x Al x As photocathode by an on-line spectral response measurement system. The spectral response curves of Ga1−x Al x As photocathodes treated by different chemical cleaning methods are measured and analyzed in detail. We use the quantum efficiency formulas to fit the experimental curves transforming from the spectral response curves, and obtain the related performance parameters such as the surface electron escape probability, the back-interface recombination velocity, the electron diffusion length, and the thickness of the etching GaAs layer. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the Ga0.37Al0.63As photocathode could be well cleaned by the solution of sulfuric acid and hydrogen peroxide. [Copyright &y& Elsevier]