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GaAs layers grown on misoriented silicon substrates are examined for defect reduction as a function of thermal annealing and degree of misorientation. These GaAs layers (3–4 μm) are grown by a two‐step metalorganic chemical vapor deposition process on Si substrates misoriented 1°, 1.5°, 2°, 3°, 4°, and 6° from (100) toward . Annealing takes place in an open tube furnace under an arsine ambient at 850 °C for one or two 30‐min cycles. Double‐crystal x‐ray rocking measurements and plan‐view and cross‐section transmission electron microscopy are used to evaluate the resulting crystal quality. Prior to annealing, all cases exhibit approximately equal defect densities with the average size of the microtwins being a function of misorientation. There also exists an anisotropy in the microtwin variant distribution in the layers. After annealing, however, the defect density is found to be dependent on the misorientation. The 1° and 2° layers, which have smaller microtwins, exhibit a greater reduction in defect density after thermal cycling than the 4° and 6° layers. The annealed 4° and 6° layers instead exhibit larger microtwins on average than the as‐grown layers. These data indicate that smaller microtwins are more likely to be annihilated through thermal cycling. In addition to the change in the microtwin structures, thermal cycling produces elongated dislocations in the 1° and 2° layers and dislocation tangles associated with the remaining microtwins in the 4° and 6° layers.