High frequency factors are associated with two main classes of unimolecular reactions: (1) reactions in which two bonds are ruptured simultaneously, and (2) certain dissociation reactions in which only one bond is broken, such as the decomposition of ethane into two methyl radicals. These high values are related to the entropy increases occurring during the rupture of the bond or bonds, and a discussion is given of the way in which the entropy may change during the process. For the reactions of type (2) the data are consistent with the view that the entropy change occurs well before the bond is completely ruptured. For reactions of both types the rates at low pressures (i.e., the rates of energization) are probably not affected by such entropy increases, so that the pressures at which transition occurs from first to second order will be abnormally high for such reactions; this point of view is supported by detailed calculations for five reactions. The entropy changes during the course of reaction are interpreted as being due to a ``softening'' of molecular vibrations in the activated state.
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