Deep levels in semi‐insulating GaAs:Cr have been detected and characterized before and after Si implantation and thermal annealing. The measurement techniques used were current transients, photoinduced current transients, capacitance transients, and photocapacitance transients. Hole emission from the Cr centers with activation energy &Dgr;ET= 0.85±0.01 eV was recorded in the semi‐insulating substrates as well as in the implanted layers. An electron trap level with &Dgr;ET= 0.52±0.01 eV and a hole trap with &Dgr;ET= 0.15±0.01 eV have been consistently observed in the implanted and annealed GaAs. The capture cross section of these traps and other traps detected in unimplanted and implanted GaAs:Cr have been determined.