The quantitative analysis of carbon in liquid‐encapsulated Czochralski (LEC) GaAs crystals has been investigated using secondary ion mass spectrometry (SIMS). Localized vibrational mode (LVM) infrared absorption measurements are calibrated by SIMS results. A LVM absorption of 1 cm−2arisen from carbon was found to be produced by a carbon concentration of 9.5±2.9×1015atoms cm−3at room temperature. This carbon LVM optical cross section is 2.7 times larger than the value which had previously been reported. Carbon concentrations in LEC GaAs crystals are estimated to be 1–3×1015cm−3or lower.