A symmetry-based, nondisplacive mechanism for the first-order B3B1 phase transition exhibited by many binary semiconductors is proposed. Using a single-molecule R3m unit cell, the energetic and dynamical features of the transformation are disclosed along a transition path characterized by the internal coordinate, the lattice constant, and the rhombohedral angle. First-principles calculations on the wide-gap semiconductor ZnO are performed to illustrate the attainments of the proposed mechanism. Computed potential energy surfaces and Bader analysis of the electronic density are used to describe the atomic rearrangements, the energy profile along the transition coordinate, and the effects of the external pressure on this profile. The geometry and energy of the transition state are determined, and the bonding details of the transformation identified. The proposed mechanism explains the change in coordination from 4 (B 3) to 6( B1), the less covalent Zn‐O bond in the B1 structure, and the transformation of ZnO from a direct-gap ( B3) to an indirect-gap ( B1) material. A theoretical formulation of the mechanisms of solidsolid transformations is currently demanded in various areas of condensed-matter physics. The microscopic mechanism and the domain growth ~many times treated by phenomenological models! 1,2 are the essential ingredients of the global kinetic description of the transition. Whereas most of the experimental and computational investigations performed on phase transitions are mainly concerned with the thermodynamics and the ~phenomenological! kinetic aspects of the process, few works consider the microscopic details of the atomic displacements from a general theoretical perspective. 3,4 At the experimental front, Knudson and Gupta 5 have lately reported a method that gives mechanistic information from picosecond time-resolved electronic spectroscopy. Coupled with ab initio calculations, the observed real-time changes in the electronic spectra have been used to propose an intermediate state during the shock-induced wurtzite-rocksalt transition in CdS. 6 More recently, Wickham et al. have shown that shape changes determined by x-ray diffraction can provide the microscopic details of the four-fold to six-fold coordinated transformation in CdSe nanocrystals. 7