We have used reflection high energy electron diffraction (RHEED) to determine the step structure on Ge and GaAs surfaces. The results show the key role that inequivalent steps on Ge substrates play in suppressing antiphase domain formation at the GaAs/Ge(100) interface. For Ge surfaces misoriented toward the  direction and exposed to an As flux, a mass migration causing a four-monolayer step periodicity is observed. If a Ga flux is then applied, a single domain of GaAs is formed. The single domain is observed at the start of growth with weak half order streaks appearing in only one azimuth. In contrast, for surfaces misoriented exactly toward the . RHEED measurements show a different distribution of multilayer step heights with no clearly predominant even or odd layer step height. GaAs grown on this surface shows a reconstruction consisting of two domains.