We discuss the derivation of the formation enthalpy and entropy of the individual native defect components of AgBr, AgCl, and NaCl, from measurements of the subsurface ionic space charge potential. The straighforward intrinsic analysis for AgBr yields a silver vacancy formation energy of (0.30±0.04)eV + (7.2 ± 2.0)kT. The NaCl analysis involves some assumptions about impurity concentrations and association, and yields a cation vacancy formation free energy of (1.24 ± 0.12) − (6.53 ± 1.4) kT. For NaCl, the formation energy is divided almost equally between the two vacancies; in AgBr and AgCl the silver interstitial formation free energy vanishes when extrapolated to the melting point.