The interaction between hydrogen plasmas and a (001) surface of GaAs grown by molecular beam epitaxy is studied by photoemission using synchrotron radiation and reflection high energy electron diffraction (RHEED). Measurements of the As(3d) and Ga(3d) core levels show that the interaction is complex and bonding to both As and Ga occurs. It is seen from RHEED measurements that plasma exposures lead to an unreconstructed and partly disordered surface. Valence band studies by angle resolved photoemission show a prominent hydrogen induced feature in the heteropolar gap at an energy 7.7 eV below the valence band maximum.