Ultraclean, integrated metal‐oxide‐semiconductor oxide fabrication has been investigated for the first time by combining (i) surface cleaning in inert ambient, (ii) wafer transfer through ultrahigh vacuum, and (iii) thermal oxidation in an ultrahigh vacuum‐based reactor. Device quality oxide structures are obtained (evidenced by dielectric breakdown characteristics for Al gate capacitors) under suitable conditions, while under other circumstances chemical mechanisms severely degrade electrical performance; even in ultraclean environments, impurity‐related Si etching reactions before oxidation degrade oxide quality, but this can be avoided by appropriate use of passivating oxide films which prevent roughness associated with etching.