An electrochemical cell Pt/AgI/Ag2 X/Pt (X=S,Se) has been used as a highly controllable source of chalcogen dimers forn‐type doping of Ga1−xAlxAs grown by molecular‐beam epitaxy (MBE). The incorporation behavior has been investigated as a function of alloy composition, growth temperature, and arsenic overpressure. At low temperatures, <600 °C, sulfur and selenium are incorporated into Ga1−xAlxAs in a facile manner. At higher temperatures, where loss of chalcogen dopant from GaAs has previously been observed, sulfur shows an enhanced stability in the aluminum‐containing alloys. Excess As4is shown to hinder the loss further. The behavior is modelled in terms of the relative stabilities of the volatile and involatile gallium and aluminum chalcogenides under MBE growth conditions.