Chemical rebonding (CR) and the pseudo-Jahn-Teller effect (PJTE) are best regarded as two distinct physical phenomena that can cause symmetry-lowering displacements of substitutional impurities in semiconductors. In the PJTE, the energy gain comes from the deep states in the gap region and occurs only when the quasidegenerate manifold of deep states is partially occupied. In contrast, in CR, the energy gain comes primarily from the "hyperdeep" localized states within and/or below the valence bands. The two phenomena can coexist, but one or the other usually dominates. In the case of substitutional oxygen in Si, the observed displacement in the <100> direction is inconsistent with the predictions of simple PJTE models and is attributed to chemical rebonding. © 1988 The American Physical Society.