The effect of growth conditions on interfacial roughness in InAs/GaAs hetcrostructures prepared by molecular beam epitaxy has been investigated using synchrotron radiation. Measurement of grazing incidence X-ray reflectivity (GIXR) and total electron yield (TEY) in the soft X-ray wavelengths are used for nondestructive characterization of interfacial roughness in these semiconductor heterojunctions. Our results indicate that a smoothing process takes place during epitaxial growth, and that the measured interfacial roughness is lowest for In-stabilized deposition on a GaAs substrate at 2° off (100), and highest with As-stabilized overgrowth on GaAs(100). The surface and interfacial roughness are practically independent of the layer thickness in the range investigated (70-600 Å). © 1990.