An infrared spectroscopic technique for the determination of substitutional carbon in GaAs films grown by metalorganic vapor phase epitaxy (MOVPE) has been developed. This technique is based on observation of the C-Ga local vibration mode in the infrared spectrum of GaAs. The attenuated total internal reflection (ATR) geometry is used to sample the GaAs films. In this technique, the sampling infrared beam is internally reflected through a Ge crystal which is sandwiched by the GaAs sample. Experimental results of the application of this technique to MOVPE grown GaAs samples are presented. These samples were grown in a horizontal reactor by using arsine and trimethyl gallium and were doped with carbon in the range of 1015-1019 atoms/cm3. The carbon content was also determined by SIMS and estimated from measurements of electrical properties. A comparison of the infrared results with these results is presented. The reflectance under ATR sampling is calculated from Fresnel's equations formulated for an absorbing heterostructure, and the experimental data are in excellent agreement with the calculation. The ATR technique is quick, non-destructive, and useful for quantifying the concentrations of carbon and other substitutional impurities in GaAs epilayers. © 1991.