Hydrogen plasma processing of GaAs and AlGaAs using an electron cyclotron resonance plasma reactor, which is vacuum-linked to a molecular-beam epitaxial (MBE) growth chamber is reported. Native oxide removal and surface cleaning of GaAs is characterized using hydrogen plasma processing, subsequent thermal Cl2 etching, and vacuum annealing. It is shown that surface reconstruction and excellent GaAs/GaAs interfaces can be achieved using these dry vacuum procedures. It is also shown that AlxGa1-xAs native oxides can be removed for 0 less-than-or-equal-to x less-than-or-equal-to 1 using hydrogen plasma processing before MBE overgrowth. The best AlGaAs/AlGaAs interfaces are obtained using low microwave power during hydrogen plasma processing. 0 and C impurities detected at these interfaces increase with higher Al composition; Si interface impurities tend to increase with higher microwave power. In general, hydrogen plasma processing is judged effective for surface preparation before MBE growth for the complete range of AlGaAs alloys.