GaAs-on-Si epitaxial crystal quality has historically been limited by a number of growth-related defects. In particular, antiphase domain boundaries (APBs) can nucleate at the GaAs/Si interface and propagate throughout the entire GaAs layer. Still little is known about how thermal processing can affect the APB density in GaAs. In this study, GaAs was grown on nominally on-axis Si(001) by metal-organic chemical vapor deposition. The effect of ex situ post-growth annealing was evaluated for a temperature range of 550-700 degrees C. It was found that upon annealing the APB density was decreased significantly. The rate of APB density decrease was found to be temperature dependent. At annealing temperatures of 650 degrees C and above, the APB density was reduced from 0.10 mu m(-1) to approximately 0.010 mu m(-1) in less than 10 min. The activation energy for APB dissolution was determined to be 3.8 eV. The mechanism of APB dissolution is discussed.