We have developed a process flow for the selective-area metalorganic vapor-phase epitaxy (MOVPE) of GaAs on Si (100) substrates. Arsenic passivation was demonstrated in the reactor environment at a temperature of 650 °C. The subsequent deposition of GaAs was carried out in a two-step process, with nucleation at 250-550 °C and second-step growth at 630 °C. High-resolution x-ray diffraction of films nucleated at 350°C or below exhibited polycrystalline grains, whereas films nucleated at 400 °C and above were single crystal. A clear c(4×4) LEED pattern of GaAs deposited on a blanket Si substrate is demonstrated for the first time. Growth on patterned Si indicates improved crystallinity as determined by XRD and plane-view TEM. The photoluminescence of SA-MOVPE GaAs on Si is shown to be nearly identical to homoepitaxial GaAs. The potential of these materials for the fabrication of high-speed devices will be discussed at the meeting.