A series of dyotropic rearrangements for dihalogenated hydrocarbons have been investigated by using the B3LYP/6-311++G(d,p) method. In all cases, an ethylenic transition state has been located. The activation energy of the basic dyotropic rearrangement of chlorine migration is calculated to be 41.9 kcal/mol. Both conjugation and hyperconjugation effects lead to a delocalization of the newly formed pi electrons in the transition state and therefore facilitate the rearrangement. For the dichlorinated cycloalkane series, it has been shown that the variation of the activation energy simply depends on the ring strain. In contrast, the dyotropic rearrangements of dichloro-cycloalkenes possess some complexity. In particular, the dyotropic reaction of 5,6-dichlorocyclohexa-1,3-diene is predicted to have an anomalous lower barrier, which can be attributed, to a great extent, to the aromaticity of the transition state. Similarly, due to the involvement of antiaromaticity in the transition states, the dyotropic migration barriers of 3,4-dichlorocyclobutene are found to be significantly higher than those of the corresponding dichlorinated cycloalkanes. Two possibly competitive processes, thermal elimination and sigmatropic migration, have been examined for some systems. The results suggest that the sigmatropic migration often possesses a lower barrier and therefore needs to be scrutinized to predict the observable dyotropic process. In addition, the dyotropic bromine migrations have also been investigated and predicted to have a much lower activation energy, indicating that the dyotropic rearrangement is more likely to occur in the dibrominated compounds. A mixed dyotropic rearrangement in which the chlorine and the bromine interchange their positions has proven to be feasible with the activation energy lying between that of the dyotropic chlorine migration and that of the dyotropic bromine migration.