We have developed a novel thermal precracking technique which has improved the electrical quality of GaAs grown using trimethylarsenic, while maintaining excellent surface morphology. Background doping is reduced by a factor of 5, and carbon incorporation is reduced by a factor of 10 or more. This method may prove useful for reducing carbon incorporation from other organometallic arsenic sources as well. Net background doping below 1016cm−3and room‐temperature electron mobilities of 4000–4500 cm2/V s have been obtained. These are the best values reported for GaAs grown using trimethylarsenic.