The free scission energy is the thermodynamic parameter that governs the contour length of wormlike micelles (WLMs). It is the contour length and the propensity to coil and entangle that determine the viscoelastic properties of this commercially important substance class. The free scission energy Delta F-sc and the associated change in enthalpy Delta H-sc and entropy Delta S-sc on scission have been determined for a mixed anionic/zwitterionic surfactant system (sodium laureth sulfate and cocamidopropyl betaine) at various salt concentrations (3-5 wt % NaCl). Both enthalpy Delta H-sc and entropy Delta S-sc changes decrease linearly with increasing NaCl concentration. At NaCl concentrations above 4 wt%, Delta S-sc even adopts negative values. The term T Delta S-sc decreases more rapidly than Delta H-sc around room temperature and causes the observed elongation of WLMs upon addition of NaCl. It is suggested that Delta S-sc is initially positive due to fewer bound counterions per surfactant molecule at end caps compared to the intact, cylindrical parts before scission, leading to a net release of ions upon scission. Negative values of Delta S-sc are attributed to hydrophobic hydration occurring at the end caps at high salt concentrations. Na-23 NMR measurements indicate the presence of immobilized ions, supporting a previously proposed ion-cloud model based on neutron scattering results.