Dopant atoms segregate to SiO2/Si(001) interfaces and are deactivated there. This can cause problems in fabrication of submicron microelectronic devices. On the basis of ab initio calculations, we propose a mechanism for donor segregation and deactivation. We argue that donor species (P and As) are trapped as threefold-coordinated atoms at interface defect sites (dangling bonds and Si vacancies) and, most significantly, in form of dopant pairs at defect-free interfaces. This pairing will dominate when dopant concentration exceeds approximately 10(19) cm(-3).