Heavily carbon-doped In0.53Ga0.47As with hole densities between 5.6 X 10(18) and 2.1 X 10(20) cm(-3) has been grown by solid source molecular beam epitaxy on InP. The dependence of carbon tetrabromide (CBr4)-induced lattice mismatch upon the atomic carbon concentration has been determined from x-ray rocking curve measurements. It has been found by secondary ion mass spectroscopy that the alloy composition is altered by the preferential etching effect of CBr4. After taking into account this compositional change, the "intrinsic" lattice contraction solely due to carbon incorporation has been obtained, which obeys Vegard＇s law.