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Carbon‐doped GaAs with carbon concentrations ranging from 2×1017 cm-3 to 2.6×1020 cm-3 has been characterized by variable temperature Hall effect measurements, secondary ion mass spectrometry (SIMS), and double‐crystal x‐ray diffraction (DCXD). The samples studied were grown by metalorganic chemical vapor deposition (MOCVD) and by metalorganic molecular beam epitaxy (MOMBE). The hole mobility is dominated by degenerate conduction for hole concentrations ≥1×1019 cm-3, and the 77 K resistivity is typically 30%–35% lower than at 300 K in these samples. The mobilities of C‐doped p+‐GaAs are found to be significantly higher than for Zn‐ or Be‐doped p+‐GaAs for doping concentrations in excess of 2×1018 cm-3. The maximum achievable hole mobilities for C‐doped material grown by the two techniques are nearly identical, indicating that neither MOCVD nor MOMBE has an inherent advantage over the other for producing low‐resistivity p‐type GaAs. SIMS analysis and Hall effect measurements reveal that the total carbon concentration, [C], is higher than the as‐grown hole concentration, p, in the most heavily doped samples. DCXD measurements show general agreement with the lattice mismatch predicted by Vegard’s law. However, for [C]≳1020 cm-3 a discrepancy between the predicted and measured mismatch suggests that partial lattice relaxation or the presence of interstitial carbon may need to be considered in order to adequately describe the lattice contraction.