Data are presented on the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/AlSb quantum wells grown by molecular‐beam epitaxy. Because both anion and cation change across an InAs/AlSb interface, it is possible to grow such wells with two different types of interfaces, one with an InSb‐like bond configuration, the other AlAs‐like. Electron mobility and concentration were found to depend very strongly on the manner in which the quantum well’s interfaces were grown, indicating that high mobilities are seen only if the bottom interface is InSb‐like. An As‐on‐Al sites antisite defect model is postulated for bottom AlAs‐like interfaces. Such antisites were used in subsequent samples as donors in modulation‐doped high‐mobility InAs/AlSb quantum wells.