Hydrogen plasma exposure of silicon‐doped Ga1−xAlxAs epilayers withx<0.37 causes a strong reduction of the free‐electron concentration in the layers. Forx<0.29, this effect is accompanied by a simultaneous increase of the electron mobility. This is interpreted, as in GaAs, in terms of a neutralization of the active silicon donors by atomic hydrogen. The neutralization efficiency of the shallow donors increases asxincreases. Forx&bartil;0.25, theD‐Xcenters are very efficiently neutralized by hydrogen and, as a consequence, the conductivity mechanisms after exposure are only governed by the remaining shallow donors. For 0.29<x<0.37, most of theD‐Xcenters are neutralized, but the electron mobility after hydrogenation is reduced.