Complex conductivity measurements of unannealed and annealed semi‐insulating GaAs crystals are reported. A model of fluctuations of well‐defined magnitude superimposed to a Gaussian distribution accounts for the &sgr;ac ∝ &ohgr;2dependence observed above 106Hz in some unannealed samples. The effect of annealing on the magnitude of long‐range potential fluctuations and on the average carrier density is analyzed.