Heavily carbon (C)‐doped GaAs grown by low‐pressure organometallic vapor phase epitaxy using carbon tetrachloride exhibits a lifetime significantly lower than that obtained by zinc doping in previous work and does not exhibit strain relaxation for thicknesses above twice the critical thickness for misfit dislocation formation (hc). GaAs:C+is shown to be thermally unstable to annealing cycles as brief as 600 °C for 4 min. The instability is manifest in conductivity, photoluminescence intensity, and lattice contraction reduction for carbon concentrations above 7×1019cm−3. This reduction is not alleviated by reducing the layer thickness belowhcor adding indium to increasehcabove the layer thickness. This suggests that eitherhcis not an adequate measure of misfit dislocation formation during annealing or misfit dislocation generation is in general not responsible for the thermal instability of GaAs:C+.