Hahn-Meitner-Institut, D-1000 Berlin 39, W. Germany Abstract The transient photoconductivity, in MBE grown silicon doped GaAs films, was studied by a contactless microwave conductivity technique. The dark carrier concentration of the investigated films varies from 4 x 1014 cm-3 to 6 x 1017 cm-3. At least two decay channels can be distinguished. An initial fast decay process (t<20 ns), which is excitation intensity dependent, followed by a slower one. The fast decay is tentativly identified with a direct electron-hole recombination and is more pronounced at high excitation intensities and in low impurity materials. The slower decay component is characterized by effective electron lifetimes ranging from 2 µs for low impurity fims up to 100 µs for high impurity samples. It can be concluded that an increase of the Si-concentration leads to an increase in the concentration of hole traps, which quenches the initial electron-hole recombination and decreases the electron decay rate in the microsecond region.