The effects of hydrogen plasma exposure on the concentration of donors in high‐purity lightly Si‐doped molecular beam epitaxial GaAs have been investigated by photothermal ionization spectroscopy, low‐temperature photoluminescence, capacitance‐voltage, and Hall‐effect measurements. Photothermal ionization measurements show that in addition to Si donors S and Ge donors are present in the original high‐purity samples. After hydrogenation, the Si donor concentration is significantly reduced with a corresponding increase in mobility. Low‐temperature photoluminescence also showed a decrease in the full width at half‐maximum of the neutral donor bound exciton line indicating that the total impurity concentration is reduced. These results provide spectroscopic evidence to support the neutralization of Si donors confirming earlier results of the effects of hydrogen plasma exposure in GaAs.