Epitaxial layers of InSb have been grown on Si substrates by molecular beam epitaxy. Room-temperature electron mobilities are 48 000 and 39 000 cm 2 /V s for 3.2 μm-thick InSb with and without a thin GaAs buffer, respectively. The corresponding carrier concentrations are 2.2×10 16 and 2.7×10 16 cm -3 . A sample with an InSb thickness of 8 μm exhibited room-temperature mobilities as high as 55 000 cm 2 /V s with carrier concentrations of about 2.0×10 16 cm -3 . A sharp band-edge transmission spectrum is observed at room temperature for the 8 μm layer.