The strength of the QS-SQ bond is investigated by studying the thermodynamics of the hydrogenation reaction using statistical mechanical relations and electron structure calculations at the DFT and MP2 levels of theory. Ten substituents, Q, have been considered. The free energy changes for these reactions which span a range in excess of 400 kJ/mol - are shown to have a linear relationship with their corresponding enthalpy changes. The disulfidicity, which is defined as the scaled relative free energy changes, provides a convenient way of describing the strength of the S-S bond. (C) 2012 Elsevier B.V. All rights reserved.