The electrical properties of carbon doped GaAs and AlGaAs were studied as a function of substrate temperature and CBr4 flux for the doping range similar to 10(18) - 10(20) cm(-3). Hall measurements indicate a strong reduction in the free carrier concentration of GaAs films grown with the same CBr4 flux at substrate temperatures above 620 degrees C. Secondary ion mass spectroscopy measurements, however, show no reduction of chemical carbon concentration. The electrical properties of GaAs:C epilayers grown on (n11)A and B surfaces, where n = 2-5, show strong dependence on crystallographic orientation. Based on these measurements, the model of free carrier concentration reduction in GaAs:C based on formation of electrically inactive C-C pairs has been proposed. In contrast, no anomalous carbon incorporation in AlGaAs has been detected for the doping range similar to 10(18) - 10(20) cm(-3) and the substrate temperature range 550-700 degrees C. The resulting material exhibits excellent transport and optical properties.