Infrared wavelength modulation absorption spectroscopy was used in the spectral region of 0.3–1.45 eV and the temperature range of 80–300 K, to study deep level impurities and defects in undoped semi‐insulating GaAs grown by the liquid encapsulated Czochralski technique. The measurements revealed two resonant type peaks with fine structure near 0.37 and 0.40 eV, as well as thresholds and plateaus at higher energies. The sensitivity of the measurements allows us to give credence to changes in the absorption coefficient at levels ∼10−3cm−1. The absorption band at 0.37 eV is interpreted as being due to the intra‐center transition between levels of accidental iron impurity, split by the crystal field. The absorption band near 0.40 eV, can be annealed out by heat treatment, and is considered to belong to a multilevel defect complex. Utilizing the photo‐quenching behavior of the absorption in the spectral region of 0.6–1.4 eV, it was shown that conventional room temperature optical absorption may give erroneous results in measuring the concentration of the EL2 levels, because of appreciable absorption due to other residual deep levels in this spectral region, as revealed by the sensitivity of the wavelength modulation technique.