We have used reflection high-energy electron diffraction to characterize the initial surface of misoriented Si (100) and then to follow the nucleation of GaAs. Measurement of the diffracted intensity along the length of the specular streak shows sharp structure due to an ordered array of steps. The initial surface contains monolayer steps. However, after exposing to an As4 flux above 650°C, the surface morphology changes to multilayer steps with four times the original period. In contrast, below 650°C, surface migration is inhibited and monolayer steps are retained. Subsequent growth of GaAs on either the monolayer- or multilayer-stepped surfaces yields single domain films. However, GaAs grown on the monolayer steps is misoriented toward the (111)A while GaAs grown on the multilayer steps is misoriented toward the (111)B.