We examine the mechanism that drives some impurities, e.g., oxygen in silicon, off the nominal substitutional site. We introduce a simple model of three atoms in a row and determine in a quantitative way the condition that can drive the central atom off center. The instability is against rebonding, not related to a Jahn-Teller distortion. The corresponding calculations for substitutional oxygen in silicon reveal that an instability can occur, depending on the form of the repulsive interactions. The oxygen atom can be stabilized in a <100> direction, as is observed experimentally, only if the neighboring silicon atoms move toward it. Otherwise, a site along a <111> direction, i.e., bonding to only one silicon atom is preferred. © 1986 The American Physical Society.