The effects of hydrogenation and subsequent annealing on unintentionally doped GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition have been characterized by capacitance‐voltage measurements, Hall effect measurements, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Significant reduction of the carrier concentration in the GaAs layers after hydrogen plasma exposure is obtained. TEM shows that the hydrogen plasma slightly etches the surface of the GaAs layers, and EDS demonstrates that the etched area becomes arsenic deficient and contains minute Ga particles. In addition, atomic hydrogen diffuses deeply along threading dislocations and microtwin interfaces into the GaAs layers and reacts with GaAs locally around the defects.