We present the results of Hall measurements on heavily carbon-doped GaAsSb epilayers grown by metalorganic chemical vapour deposition (MOVPE) on InP substrates. An extremely strong alloy scattering effect is observed in this material, dominating the Hall mobility even at doping levels in the 10(19) range. This effect is due to the very large (1 eV) valence band offset between GaAs and GaSb. Despite the strong alloy scattering, conductivities as high as 890 S/cm were observed at doping levels above 10(20) cm(-3). CCl4 and CBr4 were investigated as p-type dopants. Hole concentrations of up to 1.4 x 10(20) and 3.0 x 10(20) cm(-3) were obtained at growth temperatures of 560 degreesC and 500 degreesC, respectively. For both carbon sources, a strong reduction in growth rate and Sb incorporation rate was observed with increasing dopant concentration at 560 degreesC. Carbon incorporation was observed to increase linearly with Sb solid phase mole fraction.