An entropy model that can be used to quantitatively estimate the position of the transition state for unimolecular reaction is presented. A series of 12 isomeric reactions have been investigated to validate this model. It has been shown that the position of the transition state predicted by the entropy model (chi(s)(not equal)) is qualitatively consistent with the Hammond postulate (HP) except for the isomerizations of FSSF and CH3SH. The inconsistency for these two reactions may be well ascribed to the dissociated character of their transition states that would lead to the entropy deviating from a normal unimolecular behavior. Comparisons of chi(s)(not equal) values with other quantities characterizing the position of the transition state have also been made. (C) 2003 Elsevier B.V. All rights reserved.